Yes, they can.
Collecting disability benefits offers NO job protection whatsoever. It is FMLA, which may or may not be running concurrently with the disability benefits, that offers job protection. However, even FMLA only offers 12 weeks in most cases. And barring a state law (which PA does not have) or a bona fide, legally enforceable contract or CBA which specifically says otherwise, once FMLA has expired the employer may legally terminate, regardless of the legitimacy of the need for additional medical leave and regardless of disability benefits.
You have not posted enough information to know if FMLA applies or not and if so, whether it it still in force, but a termination while "on disability" is not illegal per se.