It’s got to be at server level
Check it out carefully, check your logs.
Someone might have installed a "nice link hijacking"
Check modified file time
and figure if you really modified each of those.

By the way do all the links consistently go visiting the same location?
If so as mentioned above there is somewhere an engine generating that behavior

None of the files have been modified. I've been able to speak to two of the customers and they were both in completely different sections of the site. One of the customers provided the exact link they clicked. One time it sent him to the earn your degree site and the other to the correct page.

Could be at the other end. Maybe something as basic as the domain name not being renewed on time and being parked. On a hosting provider forum I see occasional claims that sites have been "hijacked" that always come back to failure to renew the domain.

It could also be a spyware problem on the users' computers. I've heard of some spyware/adware that hijacks links, particularly in certain categories.