In July George H.W. Bush shaved his head for Patrick, a two year old boy with cancer.
Patrick is the sun of a secret service agent. A group of secret service agents shaved their heads to show solidarity with the little child and George H.W. Bush got inspired and he did the same.

See the news and an interview with Bush sen. and his wife:

ICCCPO chairman Kenneth Dollman was moved by the fact that Bush as a former president supported a staff member so he wrote him a letter. In return George H.W. Bush sent a very kind reply. (link)