Photo credit: Wisconsin Office of the Governor
Here's how it happened.
Walker went on to say he was fixing something in the kitchen — he doesn't say when — and hit his head on the cabinet. His wife, Tonette, repeatedly urged him to see a doctor, and when he finally did he was told his head would not be growing hair anymore in that area. He said Tonette cited this as proof that it's best to listen to your wife.
Source: Walker's bald spot becomes topic in hotly contested race. (Milwaukee Journal Sentinel, 10/23/2014)