This. One thing I've noticed is that some people simply don't know the etiquette. Not to stereotype, but most of the tri people I've ridden with have no clue about group riding protocol. Some of these people are even close friends, so I know they would have pointed out dangers (and not absent-mindedly get in the aerobars) had they thought about it. One guy (not a triathlete, but someone who rode with a friend of mine frequently) had a habit of almost throwing his bike backwards when he stood during climbs. I made a mental note to stay a bike length away. So, unless and until I am familiar with a person's riding habits, I play it safe.