reunite: passive when used as a transitive verb

Collins labels its use as [be VERB-ed + with]

Longman labels it as: Reunite is usually passive when used as a transitive verb; "be reunited with somebody" (likewise there's an entry for the idiom "be entwined (with something)", for which Wiktionary offers a note "Particularly used in attributive form entwined.")

Unlike for example gather, Oxford English Dictionary shows the entry for reunite labeled as [usually passive].

What is the reason behind using reunite in so-called "passive" even in a sentence where there's no semantic agent and meaning?

Secondly, is there a pattern that a group of verb such as reunite follow?

"Passives without an agent
Passive structures without an agent are very common. We use these structures when an agent is not important, or is unknown or obvious:

All applications must be received before 31 July.

The data was analysed and the results have just been published.

I walked to work. The car’s being repaired."

Cambridge Dictionary, passive without an agent

My comments moved to my answer:

• The couple were reunited [by circumstances or the situation or their friends]. In passive constructions there is always an implied agent of some sort. –

• They were entwined [by their love] in each other's arms. Both can be attributive: The reunited couple were not happy. The entwined couple were happy. They were reunited [by x] with their families.

• Let us reunite this family today! Not passive, active. The parents were reunited with their children by the NGO. "with" is not part of the equation necessarily.

Passives without an agent are called short be-passives by Pullum, who provides an academic explanation;

2.1.1 Short be-passives

"The by-phrase in a passive (which CGEL calls an internalized complement, because it is internal to the VP but would have been external to it if expressed as the subject of an active clause) is usually omissible, so some passives contain no counterpart of the NP that would have been the subject in the related active. When a passive clause has a by-phrase, as in (4a), I will call it a long passive. The kind that does not, as in (4b), will be called a short passive.

(4) a. The president's authority has been much diminished by recent events in Washington.
b. The president's authority has been much diminished."

Sentence b. above is what I have been saying in this answer. I would add that there is an implied passive. to be reunited with x is a short passive.

• They were reunited with their friends. [short be-passive, no apparent agent]
• They were reunited by fear with their friends. [long passive, agent: by fear]

VERSUS what Pullum calls "adjectival passives", which can be ambiguous.

2.2.4 Adjectival passives

"The term `adjectival passive' is often applied (perhaps not very felicitously) to active clauses with predicative adjective phrases in which adjective derives from the past participle of a verb and has a passive-like meaning. There is frequently an ambiguity between be passives and adjectival ones. For example, The door was locked is ambiguous: as a be passive it says that at a particular time someone took the action of locking the door, and as an adjectival passive it says that during some past time period the door was in its locked state. Since the complement in this kind of clause is an adjective phrase, verbs other than be can be used (The door seemed locked, as far as I could tell), and so can adjectives derived with the negative prefix un- (The island was uninhabited by humans)."

My answer contains the example: The room was painted. versus The room was unpainted. versus The room was painted [by some agent].

So, the scholarship in this case tallies with the what the Cambridge Dictionary states (given above).

The paper makes for good reading. I recommend it.

All bolding is mine.