Is this a result of server side animations?

[BerntA]

In our new game we've experienced a strange lag spike for a long time, whenever we'd spawn too many npcs in our multiplayer game it would spam the console: "WARNING! User command buffer overflow(4191 4192), last cmd was 3982 bits long" but only if these zombies were networking = if they were visible in the PVS. So in test maps this would occur pretty much always.
However when we changed the default HL2 animations we never had this issue again?
Why is that?

Screenshots:

With the new animations we have no lag issues, the ping goes up as we add more npcs but it doesn't cause extreme lag spikes:

With the old default HL2 animations we kept getting the user command buffer overflow issue: