What are the causes (and cures) for bridge experts' mistakes?
First, let's be clear on terms - no world class player makes the mistakes that you or I make: such as failing to count the hand or forgetting last week's amendment to our bidding system. The mistakes they make are far more subtle, and would be regarded as merely second-best actions by you or I, if we were even capable of analyzing the best action. 

The bidding mistakes by experts are of forgetting a conversation multiple years earlier about the particular situation that has again come up, for only the second time in our long partnership. They are failing to draw the sub-sub-sub inference from the failure of an opponent to act in the bidding. 

That being said, ultimately the source of all errors is human imperfection - and as we all are imperfect in different ways, the errors of world class bridge players arise from different imperfections. Barry Crane was infamous for refusing to allow many of his partners to eat between sessions, lest an overfull stomach interfere with clear thought in the first rounds afterwards; yet others of his partners regularly enjoyed between-session dinners as his partner.

Eddie Kantar's mis-defence on board 92 of the 96 board final in the 1975 Bermuda Bowl is an example of such unique human imperfection: failing to false card the CK from KT doubleton as West, when dummy comes down in the North with CAQ. You or I would likely never even note that the falsecard will induce a strong declarer to go down, or only long after the session with pen and paper in hand.