Crank case nut falling out?

The other day my crank case nut (dunno if that's what it's called, the nut/plug or whatever on the crank case, left hand side) decided to fall out on the way home from work. So was left with oil covered lower jeans and boot. Luckily was about 50 yards from the bike garage I bought the bike from a week ago so just coasted in. They've replaced it and it all seems fine. just wondering what could cause this? Maybe they didn't put it in very well as they had only just performed a service on the bike?