There are institutional....well, not reasons, but causes, for this. Reason would imply some merit to this decision, which will help various parties but ultimately hurt the undergraduates, who we are ostensibly there to teach. I think administration forgets that sometimes. A university is not a graduate student production facility. Today's TAs are tomorrow's (laid off) adjuncts.
The union is pissed off. We'll see what we can actually do. Nothing that will save the people cut this year, though. They're casualties of an administrative war on the adjuncts.