Manuscript status changed to editor assigned after under review, what could be the reason? Recently I submitted minor revision to springer journal. For 3-4 days, the status of my manuscript status was showing to be Editor Assigned and after that status changed to Under Review. Now status of manuscript is changed to Editor Assigned again. What is meaning of this change ?
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.