Sudden increase in resistance Ramptest

Today I did my first Ramptest and I had a strange problem.

first of all: I know how a ramp test works and that it increases resistance every 1 minute:)
Everything perfectly fine when all of a sudden the resistance increases tot a point where you can’t turn the pedals. This all within the 1 minute @ 400w. And 420 everything was fine again and at 440 it happend again.
Any ideas on this?