Was the film fresh?(posted 8275 days ago)
HP5 was replaced by HP5plus some time ago, so if it really is plain HP5, then the chances are it's out-of-date stock. Stale or badly stored film can get excessively grainy, especially fast films like HP5.
The other thing to check is that it really is grain and not reticulation, caused by a drastic change in temperature from, say, the developer to the stop bath, or from fixer to wash water.