Here is the answer, directly from Tranquill.
by rockmcd (2018-03-09 19:16:34)

In reply to: Why was Tranquil moved from rover?  posted by PattyMulligan


“It was coach Kelly and a general understanding that was what I was going to play in the NFL,” Tranquill said. “… I knew in mind I need development at that position, so it was a general understanding that [the Irish coaches] had gotten feedback from NFL teams, I had obviously gotten feedback from NFL teams, and we all understood this is what I needed to develop at.”

“It was just a decision of do I want to go and get that development in the league, or do I want to come back and develop under coach,” Tranquill said. “I think the best decision for me was to come back and develop under coach Lea.”