Yaliskah I suggested it a few times, guess I will repeat it.
Why people zerg? Cause from one hand, as undergeared and with not the best spec, numbers is the way to win. And from other, solo/small numbers roaming the same undergeared player has nothing to gain (he isn't likely to get kills), while will loose renown from zerg kills/resource delivering. Nothing to gain + things to loose from not zerging= zerging it is.
What is needed is a reward for going solo/small scale, and for moving from main zone to other zones, and by reward I mean personal and not faction wise. As in, not "if your side spreads in fort phase 2, your side has a better winning chance at stage 3", but "if you leave zerg/ move to offzone, you personally is likely to get extra renown/rewards"- since side reward results in "let the other guy spread and deal with premades, I'm staying here", while personal rewards results in "ohh shinies for me, time to go".
If you look at the current reward system, it is actually one of the main reasons for zerg. Reworked keep bag system= no reason to leave main zone, since your chance to get bag is the same whatever your side in the zone has 20 people or 200. Only real rvr reward which matters is invader in forts= no reason to leave main zone, since zerging to fort is the best way to get invader.