The start of June has typically signaled the start of promotion season in Minor League baseball. Two months at a level is a pretty good benchmark for many guys – everyday players are generally approaching 200 PAs, starting pitchers have 8-10 starts under their belts, and relievers are often 25+ innings into their campaigns. And mid-month is the draft, the end of Extended Spring Training, the start of short season/rookie leagues, and the mid-point of the five month full-season leagues. So lots of opportunity for movement.
In many cases, what it amounts to is an organizational question, not necessarily a prospect question.