Not sure exactly what's happening based on your description, but if your entire browser window is scrolling down then maybe a key (down arrow) is stuck on your keyboard?

Anyway, I don't think there's anything wrong with "casual groups" if that's what everybody agrees to. It can be frustrating to have to slow down one's pace to wait for other people, and I imagine it's equally frustrating for them to try to hurry unnaturally. Granted, this may not work for close-knit groups or those whose members aren't sufficiently self-sufficient, so to speak. Just put the slower people in the front to set the pace of the group and suffer behind them--it's your fault for planning it out that way ;-)

However, once you plan it, I agree with phat you need to stick to your plan and not separate the group.

I consider a whistle an indispensable part of my gear (when hiking or scuba diving). I think it will carry farther than a voice and is an unnatural, high-pitched sound that almost everyone will associate with people needing help.

--camelman