After a certain amount of time or deployments, a Sunderer will stop being enterable or usable. This has happened to nearly everyone who has ever pulled a Sunderer in my outfit since last patch.
Also applies to other vehicles as well. Fun thing is my sundy became invisible to my squadmates when it became "unenterable". They didn't see it, and if they tried to go through it, they'd receive a high amount of damage, often fatal.