Exception to the word order S-V-O

You are mistaken. These sentences

• Behind him was Sam.
• In a field was a big group.
• To the left was a row of barracks.
• Next to the corner was a woman.

are not exceptions to Subject-Verb-Object word order. Nor are they ungrammatical.

To start with, there are no Objects in any of them; they're all intransitive.

Second, they're all a very specific kind of intransitive sentence: they all have locative
predicates (behind him, in a field, to the left, next to the corner). All predicates that aren't
verbs require some form of be as an auxiliary verb, -- was, in the past tense here.

Third, most sentences with a main verb be and a predicate phrase can be reversed:

• [The doctor I mentioned] is [Sam]. ~ [Sam] is [the doctor I mentioned].
  (equative construction)
• [The ceremony] was [at 1:30 pm]. ~ [(At) 1:30 pm] was [the ceremony].
  (temporal construction)
• [The statue] will be [in the center]. ~ [In the center] will be [the statue].
  (locative construction)

-- though not all; predicate adjective constructions don't reverse very well, for instance:

• Sam is tired of saying this, but not *Tired of saying this is Sam.
• Helen is usually sleepy at 4 pm, but not *Usually sleepy at 4 pm is Helen.

An exceptional phrase order

Although it's actually very common, the Original Poster is correct that this is in some sense an exceptional phrase order (perhaps a better term is a non-canonical phrase order). However the phrases:

• behind him
• in a field
• to the left
• next to the Corner

are Locative Complements, not Objects. Normally, we expect to see the Subject occurring before the Predicator (the verb), and then after the Predicator we expect to see the Complement. This Complement might be an Object, a Predicative Complement, a Locative Complement or any other type of Complement - but its usual position is after the verb.

Because English does not have any case to show which phrases constitute Subjects, and which constitute Complements, phrase order is more important than in other languages which do have case. In English the following sentences mean different things:

• Bob ate the chicken.
• The chicken ate Bob.

The reason they mean something different is that the order of the phrases is different even though the words themselves are the same.

Information packaging

However, phrase order in English is not, in fact, strictly fixed. There are many times that we need or want to change the order of the phrases in a sentence to make it easier to process. The most common reason that we do this is so that important things that we haven't talked about yet appear at the end of the sentence - and not at the beginning. If we put things that we haven't talked about yet at the beginning (especially if these are indefinite noun phrases), then listeners find the sentences difficult to process. They find this especially awkward if the phrase at the end of the sentence is something that we have talked about. We prefer things that we have already talked about to appear at the beginning of the sentence, where they link back to the previous discourse. Consider the following pairs of sentences:

1. a. I've been studying the Mona Lisa. Leonardo Da Vinci painted the Mona Lisa.

   b. I've been studying the Mona Lisa. The Mona Lisa was painted by Leonardo Da Vinci.

The (b) example here is better. The first is rather stilted. The reason the passive sentence works well is that the Mona Lisa links back to the first sentence, whilst Da Vinci, which is new to the discourse, takes the prominent position at the end.

The study of the different ways that we decide to order the information in a sentence is termed information packaging.

Subject-Complement inversion

The phrase order seen in the Original Poster's examples is known as Subject-Complement inversion. It happens very often when the Subject is someone new to the conversation, especially if represented by an indefinite noun phrase. Now, although we don't like to put things we haven't talked about at the beginning of a sentence, this seems to be less important if they are locative or temporal phrases. In the Original Poster's examples the phrases which appear at the front are all Locative Complements. When these Complements of the verb BE get fronted like this, the Subject phrase switches places with it and appears at the end of the sentence. The end of the sentence is the part that receives the most focus. So the speaker has now put the brand new and therefore interesting information at the end where it carries more emphasis.

Notice that, apart from Sam in the first example, the Subjects of these sentences are all indefinite noun phrases:

• a big group
• a session of barracks
• a woman

Now if these were definite noun phrases that we had talked about before the sentences would not work:

• There was a big group there. In a field was the big group. (not good)
• As I approached the gate I noticed a session of barracks. As I went through the gate on my left was the session of barracks. (not good)
• I spotted a woman. Next to the corner was the woman. (not good)

The sentence with Sam will also be bad if we have already talked about them:

• I saw Sam at the party when I was speaking with Bertha. Behind her was Sam. (not good)

Subject-complement inversion happens with the verb BE. It can happen with other verbs but it is much less common:

• Up the street came an angry looking policemen.

The Complements which get inverted with the Subject are usually locative or temporal phrases. We can't usually do this type of inversion with Predicative Complements:

• *Out of sorts was Bob.
• *Sad was Bill.

The Original Poster's observation

These examples do indeed show an exceptional phrase order (we call this a non-canonical phrase order). However, such sentences are both grammatical and very common. We use sentences like these with the verb BE when the Subject represents an entity which is new to the current discourse. If we use it for the wrong reasons, our sentences may seem ungrammatical.