Many things perhaps or maybe the idle is just set a little low. So when the engine is put under load the idle speed is forced down too far. Contributing to this is when the engine oil is cold there is a lot of aditional drag on the engine to start with.

Then there is the aspect of running on the wvo. When the system is cold it could contribute to the issue. Much thicker viscosity to the fuel for starters. It in my opinion takes quite awhile to clear all the vegatable oil from the system before shutdown on a two tank system.