I don't know. It never happened to me, and neither to my eldest daughter (10 yo). She loves math, and her school teaches it particularly well, IMHO. But I understand that it happens to many kids, even to some of her classmates. Some hypothesis and ideas.
1. Sometimes math is taught by people that does not understand it, that teaches algorithms and memorization instead of ideas. Many parents are guilty in this: I have seen other parents evaluate the level of "advance" of their children by the number of things they "know" (e.g. my child knows more or less operations than yours, etc,). This has very little to do with maths; given enough time, a parrot could repeat a multiplication table, without having any knowledge.
2. Math is sometimes abstract and difficult, and requires time and patience. If you read slowly, you still can read. If you do not get something of math, most of the time you get the idea that you know nothing.
3. Many parents, even if brilliant in other fields, suck at maths. I know lawyers and artists that have problems with simple proportions or geometry. And they fear maths, or consider it useless, and that attitude permeates to their children (If I had languaje skills to match the maths skills of my non-scientist friends, I would be able to read at a 5-year old level, and that would be unacceptable).