Is that really the most likely?

I'd suggest it's much more likely that they tripped some automated spam protection or a moderator thought they looked suspicious, than there's a conspiracy against the company by a competitor.

That conspiracy would be of a high reputational risk to Facebook and the competitor, and would have a paper-trail in contracts that guarantee Facebook that ad revenue for the deplatforming.

It seems highly unlikely to me, as much as I believe Facebook to have poor moral standing.

Sounds like something that could be tortious interference but how could it ever be proven unless someone on the inside talks? The absoluteness of the communication cutoff certainly makes it seem like Facebook has something they want to ensure doesn't have any possibility of accidentally leaking.

This is not likely at all.

That is subject to a severe anti-trust litigation