According to my little experience, points and cash credits may be decreased due to any of these reasons.
1) When a forum thread to which you responded/you posted is deleted. Here, points and cash credits (if allotted) are decreased after deletion.
2) When a resource response you posted is reviewed. Here, points are decreased and cash credits are increased.
3) When an ask expert question to which you answered/you posted is deleted. Here, points and cash credits (if allotted) are decreased. This is an imaginary case and I didn't come across this case anytime.
4) When an ask expert answer you posted is posted twice due to internet error etc,. Here, points and cash credits are decreased. One of my friends is a member of ISC. He posted some ask expert answers. After two days, he was given points and cash credits for those answers. After a day, four points and 4 cash credits were reduced. The reason behind it is the same answer being posted twice due to internet error etc,. and the points and cash credits to the duplicate answer were reduced the next day though they were given to both the answers initially. I think that this may be the problem for you too. Please check if there are any such duplicate answers.