There is also the possibility, given his physical problems, he wanted to find a team that would allow him to JUST play centerfield and not come up and make those tackles.
Houston and the Niners fit that bill.
The other 10 players will be responsible for stopping the run and Ed will upgrade their secondary against the pass.
Houston is great against the run and gets after the quarterback. The ball will come out earlier and that will be playing into Ed's best remaining physical abilities.