It depends, was the cap dislodged from the wire, or was the cap off the spark plug itself? Possibly they did not push it on all the way during the PDI. The routing could be wrong, and pull on the wire when you go over hard bumps. You can get the NGK plug caps for about $10 and they work great, when you push the plug cap on it should make a really nice "zzzzzt" ratchet sound and take some effort to pull it off.