Lots of possibilities here.

I would really encourage you to track down the exact cause of this for one (ancillary) reason: There are many in this forum who strive to get the monster grain that you accidentally managed to get. If you figure out how to repeat this (film/exposure/development, etc.), there will be many grateful photographers out there in your debt!

I, for one, like no grain at all, but appreciate the gritty look in others' work.

As for diagnosing your problem; do check your shutter and meter to find out if overexposure is really a factor. If not, then overdevelopment looks likely.

However, if you have underdeveloped negs and are increasing contrast while scanning, that may be a large factor in the grain. Does an analogue print show this grain? If so, then it's not a scanning artifact.

Good luck,