Origin of "kicker" in reference to unexpected situation or detail

Solution 1:

Kicker, as in "here's the kicker", likely comes from poker. The kicker is a card used to break ties in hands of the same rank. The origin of that card's name (and how it is related to other kick idioms) is more difficult to determine.

Ngrams, however dubious, seems to suggest that the idiom is relatively recent:

Poker has a rich history of slang, and several other objective cards have specific names such as river, turn, et al. Other poker related idioms include up the ante, play your cards right, raise the stakes, wild card, and others.

Solution 2:

It's hard to say, but it seems to be a relatively recent phrase, originating in the US and dating back to the second world war.

Citations

• The Field artillery journal: Volume 32, Field Artillery Association (U.S.), 1942:

The Italians rose promptly - all fifty - the captain at the head. Easy capture, wasn't it? The kicker is that a few minutes later they saw another Italian soldier coming to their lines, holding his pants. He was admitted and had to explain. This was his explanation: “Being the orderly of the captain you have captured, ...

• Billboard magazine, 14 Oct 1944:

But the real kicker in the story came in when it was learned that Atlantic Refining, thru Ayer, has a contract to broadcast the games of another team over WTAM in Cleveland. Consequently, It is obvious that it will do all it can to cut ...

• The best from Yank: the army weekly, Council on Books in Wartime, 1945:

for the evening, a black "velvet sheath, outlined in jet at its shallow decolletage." That's where the kicker is, in that "decolletage" jive. That means it's cut away, all the way down the front.

• The New York times film reviews: Volume 4, 1949:

Robert Walker plays the wastrel with almost as much authority as Mr. Lancaster does his protector. The only real kicker in the story, however, comes from your own speculation as to how long Mr. Lancaster will put up ..

(Some later ones: 1952, 1953, 1954, 1955, 1958.)

Note: most of these are snippets so Google Books' dates cannot be easily confirmed. One 1944 is full view and confirmed.

Definitions

Could this be from US army slang or a journalism term?

According to The Probert Encyclopaedia:

Kicker is American and Canadian slang for a hidden and disadvantageous factor, such as a clause in a contract.
Kicker is American slang for a clincher.
Kicker is American slang for something exciting.
Kicker is American slang for loan fees.
Kicker is poker slang for the card in one's hand used to decide a tie, the highest non-scoring hard in one's hand.

There's quite a few like "the real kicker in the story" and Journalism.co.uk's glossary says:

Kicker - The first sentence or first few words of a story's lead, set in a font size larger than the body text of the story.