Most likely, you have your camera setting to free-look, common mistake! It automatically forces you to free-look, instead of just enabling it. Check all Camera Settings and vehicle settings.

I have the exact same problem, with the freelook option disabled.
The problem exists with all kinds of vehicles, land and air and seems to occur mostly after playing for a long time (several hours) on the same server. The only solution I found so far is re-joining the server.

Once the bug shows up, the view also shows weird behaviour in other situations. For example as a passenger in any vehicle the view zooms in and out in quick succession or when on foot, the weapon is shaking weirdly (too quickly, but not much) every few seconds.

So far i only noticed the issue on king of the hill servers (which is the only scenario that I play multiple hours on the same server)

@Drakeziel, that's not the issue. I have the same problem sometimes, but it is very rare. It appears with vehicles of any kind and also static weapons. I would describe it as if you're a co-pilot without having taken control, but in a vehicle that doesn't have a co-pilot seat at all. No mouse steering, no weapon control and high mouse sensitivity.

same problem for me and reconnecting to server did help.

i didnt play for some months and today sitting in a pawnee it did happen again. still not solved. free look in vehicle is already disabled in game settins. i never had to toch this setting. i was playing almost 2 hours when this happend. i did fly many birds and pawnees before the free look bug occured ....