"Under Editor Evaluation" to "Under Review"
My guess is that the editor felt the review(s) received were not sufficient to make a decision on. You can get some extra insight if you know how long the paper was in each stage. If the paper was under review for a long time (depending on how long it typically takes to review a paper in your field) and then went to under editor evaluation for a few weeks before reverting back to under review, that would be indirect evidence for this guess.

If this guess is wrong, then the next most likely thing is probably a more serious breakdown in the review process, e.g. the journal swapped editors responsible for the article. Either way, unless you want to withdraw the paper, there's nothing to do except wait.