random password that contains an html tag fails to log in

Is there a reason why a randomly generated password, if it contains something resembling an html tag (i.e. something like "hunter<*(&#@sqe/>2"), would be usable to register with a website, but not be usable to log in? What kind of problem on the back end would that be indicative of?

This isn't a problem with my own code, but rather with a website I'm trying to use. I have basically zero experience with web dev (and it's their job to fix the problem), but I'm curious what this means.