This is just a guess on my part. I think some requester's use blocks too freely. Instead of putting an exclusion qual for a specific demographic, already participated, etc. they block you because they don't know what they are doing. I think turkers that have a certain % of blocks can lose masters or don't qualify to get it. Don't know if I am right or wrong, like I said, just a guess...Who knows, they took it away no warning or anything. I have 700k hits approved with a 99.8 approval rating so I really don't know why they decided I wasn't fit to be a master.
Edit: I am not suggesting that this happened to you, just quoted you for a point of reference.