...Mathews does pull him down last but there was a reason he did that. The DB can be clearly seen from both angles using his forearm to push/drive Mathews away from the ball and out of bounds. Once Mathews steps out of bounds he became an inelgible reciever and at that point he resorts to pulling the DB with him hoping the ref would call something.

I've seen it from the perspective/viewpoint of the QB/LOS and now from this angle and neither vantage points make it 100% conclusive but I'm 80% convinced that the DB interfered first and got away with it and Mathews reacted out of frustration. Actually, seeing how desperately the yewt DBs were grabbing, pushing, holding and PIing all game long I think I'm more than 80% certain that it's the way it looks. The description of events where the yewt DB is pushing Mathews away from the ball first and Mathews grabs him as he's falling out of bounds makes perfect sense, that was the Yewt gameplan for covering BYU's WRs and the gameplan for the crooked yewt Pac12 officials who let it go all game long (we already saw this dishonest crew cheat the Wisconsin Badgers out of a game vs the Sun Devils earlier in the year on National TV and then later get reprimanded by the conference).

The puzzle comes together once you get all the particular pieces on the table.