There are several possible reasons. It could be due to a misunderstanding about a prophecy or a magical object. For instance, if Ron accidentally breaks a magical heirloom that Harry believes could be important in fighting Voldemort, Harry might be so angry that he ignores Ron. Also, it could be related to Ron's family. If Ron's family does something that Harry disapproves of, like siding with the Ministry when Harry thinks they're wrong, Harry could start ignoring Ron.