I almost always see the opposite, probably because I have only ever played IB solo queue. First team going for B gets steam rolled right back off it because their team is fragmented, and then the team that re takes as a group, tends to hold it for a while. Bum rushing B with randoms means very little in the long run. 1 on A and 5 on B is a party team chat strategy, so I don't know why people are expecting it to happen with randoms. Might as well move cap and move as a group. Randoms gonna random.
Oh with randoms, anything goes. You should queue up with a GAF IB train. We went undefeated yesterday! I think our biggest win was by something crazy like 11,000 points.