Rear differential

I took my car in to get an oil change and the guy tells
me I have a rear differential leak and he'll fix it by
replacing the pinion seal and differential cover seal
and replace the gear lube. He did the job and it was
my first time driving the car, I get about a mile and a
half down the road and the rear end feels like it jerked
and locked up on me. I get it towed back to his shop
and he says my pinion bearing is shredded and shot
so it needs replaced. He wants to charge me more
money for the repair. Could this be something he
caused? Keep in mind my car was running fine with
no problems and no rear end noise when I took it in.
Please help, thanks!