How to Stop GFCI tripping when current drops too quickly?
You might try a snubber between the power strip and the GFCI, to eliminate any electromagnetic interference produced by suddenly switching off the pumps.

Alternatively, trying a power strip that doesn't have surge protection could solve the problem.  Surge suppressors typically route surges to the grounding conductor, which could cause upstream GFCIs to trip (assuming the surge is large enough).