Answered in another thread, but I'll repeat it.
They are going for the evaluation. On the off chance that either gets a first round grade, I'm sure they'll stay. Bowers is probably more likely than English to go pro (and more likely to get a good draft grade), but really it's more for the experience than anything. All players get one free entry as long as they don't hire an agent. I'm surprised more players don't do it.
English did say he was almost certain to leave Mizzou if Anderson came back. Apparently that relationship had really deteriorated over the last year. His entering the draft may have been related to that. I don't know when exactly he filled out the paperwork.