It's the same as with RG3. SEA, MIA, CLE and WAS were desperate to fix their QB issues. MIA and SEA were out b/c that was too far to move, and so WAS and CLE both bid a ton.
Manning is at DEN or TEN, and Flynn is at either MIA or SEA. So CLE and at least one team need a QB. They have to do something. CLE will take Tannehill at #4, b/c they know MIA or SEA will take him if they try and trade too far down.
Tanny at 4, I could only imagine how pissed of Browns fans would be for such a reach.