Found an error in the console:
L 01/26/2015 [SM]  Line 72, D:\servercsgo_unlimited\csgo\addons\sourcemod \scripting\inf_slowdown.sp::OnTakeDamagePost( )
L 01/26/2015 [SM] Native "ZR_IsClientZombie" reported: Plugin owning this native is currently paused.
Just noticed that the zombies too much scatter in different directions after the explosion of the grenade. If the grenade explodes behind zombies, there is a possibility that one of the zombies can throw into the crowd.