It's college football. Every year, you are supposed to lose about 40% of your starters and a higher percentage of your top players. That is normal attrition made up for by everyone else on the roster stepping up.
The only thing that was unusual about the 2011 to 2012 transition from a personnel standpoint was the unfortunate series of events to anyone who might have competed with McCombs to start at TB. Frey leaving for a different academic opportunity, DJ only getting a few snaps all year, Meme not recognizing the opportunity and leaving early and Hyppolite losing so much time to injury that he never got in the mix. The rest of the attrition was normal.
You know, I don't remember discussing this much during the regular season but it really ended up being a big deal that Meme didn't come back for his 5th year. He left when he was well behind Todman, DJ and Frey on the depth chart, but he could have ended up at least sharing time with Lyle. Oh well.