As a junior auror, Percival's mentor was an absolute arsehole and he didn't like Graves one bit.

Every opportunity he had he'd belittle him, scream abuse, physically assault him on occasions. Percival just had to grin and bear it until he managed to get himself promoted.
Everything got better after that. His mentor is transferred/retires and he carries on his merry way to the top job, the memory of junior Auror Graves fading in the minds of those there to witness it.

So everyone's a bit taken aback when some grumpy bastard stumbles across their path, digs into Percival and he lets him.