I have to agree it sounds like most times its keeper error. But it also sounds like, you have some aggressive snakes or that they can be tempermental at times.
I do agree sometimes tagging occurs with younger snakes as well that has nothing related to feeding. However, I have heard that snakes do become used to and routinely seek out getting fed more regularly and aggressively when being strikely fed in the enclosure.
I also know that some keepers leave their snake alone and don't handle as much as they should, only opening the enclosure when its feeding time. I do believe handling your snake more and when feeding in the enclosure will decrease the possibility of being tagged with "mistaken identity" as one forum member put it.