That's my first thought. Surprised you couldn't hear him in the next block when it happened. Just keep an eye on it and make sure it doesn't get worse and that he isn't getting the line caught under his knee or foot even so it can get jerked. One of the easiest ways to jerk it is to have the collection bag attached to a chair or wheel chair and forget that it's there. Talk about inventing new swear words, that'll do it.