I think the theory is that Romney enters to further siphon off and split the delegates in order to prevent (or make it harder) for Trump to hit the number he needs to clinch it before the convention, thus forcing a brokered convention.

The problem is, of course, who says he's going to siphon off any votes from Trump, especially after he led the attack against him.

Another theory I have is that he makes himself technically eligiable so that he can be considered in a brokered convention that ends up brokered through no design of his own...with him being the only palatable adult in the room the the establishment GOP can rally behind.

Problem there is Kasich already fills a pretty good Romney-esque roll in a situation like this, but has the added benefit of actually been in the race for a while with people voting for him.

I don't know...it's very odd. And finally, nothing may come of this filing at all. Probably most likely scenario at the moment, but it's a really interesting filing nonetheless, especially combined with the speculation that he's prepared budget numbers to run as well.