A steering box seal that broke badly enough to let the PSF dump out like that is not common, but it happens. The good news is your PS system now has a fresh load of PS fluid. Hopefully you went with synthetic which won't boil as easily on a tough trail which makes steering difficult.
Here's a pic of my aftermarket Tommy Lee extra-heavy duty steering box dumping its load of PS fluid half-way up Sledgehammer in Johnson Valley. So yep, it happens.