that cap does not have a good positive locking feel to it, tie it on!
I wlll now.

 

Normally, you would only get pressure in the reservoir, if you turn the steering wheel when the vehicle is not running. The cap has a lock in, when you turn it clockwise. If it was just sitting on the reservoir, not locked in, then it could be possible for a good bump in the road to knock it off. I have seen this happen many times on vehicles, and many times I found the cap lodged below the reservoir.