Manuscript status changed to editor assigned after under review, what could be the reason?
The first mission of the editor is to assign reviewers and take a decision depending on their return. Hence the first Editor Assigned to Under Review. The reason with it is now Editor Assigned can be due to several reasons. This depends a lot on the manuscript management system, and the editorial practices.

Either:


The editor has already received the necessary  reviews, and is taking a decision (perhaps taking another look at your paper)
The editor has not received the necessary reviews.


The second case may be caused by different situations. Even with a "minor revision", it may happen that:


not enough of the first round reviewers have responded. Some journals impose a minimum. So he is looking for another reviewer (or doing the review himself), as suggested by @Buzz
the 2nd round raised more than minor concerns. Sometimes, after the first round, a reviewer notices sometimes more important, and can ask for a major revision (or a reject). So the editor may add another reviewer, to settle the case. 


The latter happened to me once as a guest editor. ED1 said major, ED2 said minor. After round 2, ED1 said OK, ED2 said reject. A very painful situation  for everybody.

I wish you are just in the editor decision frame.