A similar problem I've had - and I can't remember exactly how it happened - is when some of the party enter combat, but those who don't leave their assigned places and wander around to find each other. I think it might be when I make a surprise attack with someone who isn't the party leader.
I'd argue that when combat starts, the player should have the option to roll initiative for all their characters. It's annoying enough when a high-initiative character doesn't join the first roll, and so pretty much doesn't get to join in that first round at all
