Hayward 3/4 hp Pump

I use 2 Hayward 3/4 hp Super II pumps with my Spa.  One pump is for circulation and the other is for the jets.  The Spa became operational 3 years ago and since that time 2 of the plastic lids on the jet pump have cracked and 4 lids on the circulation pump have cracked.  Our pool maintenance people have been unable to explain why the lids are cracking.  Does anyone have any thoughts as to why these lids are consistently cracking?