Yes, another vote for the compression pants.

Sometimes I find it comes from pulling the cuffs down around my socks, so there are opposing push-pull forces between the fabrics, and the area over the knee remains tight.

Riding the hard-packed trail may also have contributed a little -- I find there is a tendency to coast over rocks and ruts just slightly off the seat, and if you do that with the pedals level (as in standard MTB practice over rough ground), and the affected knee was on the forward knee, you may have strained the ligaments above the patella.

Finally, there is a slight case that you have an imbalance between the muscles that make up your quads, strengthwise. You may need to do some exercises to strengthen, likely the inner quad muscles.

Once you have it, it does take some time to overcome it. Rest, and icing should help some.