Nope head banging is not strange at all. Read up about it. Many babies (pre-language abilities) get frustrated and bang their heads out of anger/frustration. It's normal in most cases and kids usually outgrow it. My ped. said it's a self-soothing behavior. A bit counter intuitive but normal nonetheless. My 12 month old bangs her head back in her high chair when she's pissed all the time and sometimes bangs her head into the mattress when she's upset.
(computer sucks at work can't make link clicky sorry)