I played on two servers in Classic. I never saw general races until patch 1.12, which is when xserver BGs went in. From what I gathered at the time, it started becoming common on some (but not nearly all) servers in 1.11 after they removed many NPCs and made the remainder weaker.
NPCs will assist you in there as if you were an aggro NPC that ran to more NPCs, so you can attack/be attacked then run to them then they would assist you and run at the enemy. So a handful of defenders could hold off the initial charge into the base by attacking then running into NPCs. In the horde base in particular, I used to run up both towers (which used to have strong melee NPCs inside and were close together) and get a bunch of NPCs aggro on them at once. If you charge the bases right from the start, you have ZERO graveyards to spawn at, so if you die you end up on the other side of the map and they have a large army in the way if they want to get back.