TacX Bushido: abrupt resistance change

Hi Zwifters

I am using a TacX Bushido smart with Zwift and have the following curious problem:

Especially if I am pedaling hard for a short stretch (for me hard is a full power 800W stretch), during pedaling I suddenly feel a short and unexpected burst of high resistance from the trainer - for me it is curious because I do not expect resistance from the trainer, the ground is level and I am alone..

As a consequence my “sprint” gets a blow and I am frustrated. If I look at the power output, indeed during this resistance burst there is a power peak.

Anyone with an explanation?