Hi Robert,
I had a similar surprise my first day back on skis, which was a real surprise. I grew up ski racing, competed successfully at the top level in Freestyle, and have been teaching full time out west for the past 45 years. I am usually on skis 8 hours a day for 140 to 150 days straight every season. At exactly six months after my resurfacing, I hiked up our mountain just before opening day of the season excited to try out my new hip. I started to make my first turn, and careened all the way across the run into the trees with no control; yikes! I stood there for a few minutes and gave myself a good pep talk. I then skied down the rest of the run with no problems, but very slow with a lot of technical awareness. When the lifts opened a couple of days later, I spent the first few hours on the beginner slope. By the end of that day, I was agressively skiing steeps in deep chopped powder and feeling pretty good. I will say that for me, the key was really focusing on precise technique with very focused body awareness. I hate skiing on auto-pilot anyway, but coming back from the resurfacing was a real study in precise technical awareness.
My only suggestion is that if you have not been a precise technician in the past, this would be a great opportunity to delve into that, rather than skiing on muscle memory. For me, that fits with how I think about skiing anyway, so it might not work for you, but it might explain those early falls.