Realistically, we would normally still be fighting these guys when these cutthroats arrived. The fight was only one round! That's a major beat down for it to last only one round (on both sides as BAG almost took out Riven, thankyouverymuch).
I expected them to be a second wave. We also don't know that they are going for reinforcements. They might way to rough us up find out what happened first. Of course, as soon as we cut one down and someone calls out, "Call for help!" we are pretty much hosed.
Riven is not going to willingly back down from a fight unless it is somehow for the Greater Good TM. If the rest of the group is telling him, "It's time to go" he'll probably listen. He's pissed off there was a beat down and he didn't even get to help!