How about snow? Or an avalanche? Snow could get really deep on the south side of a ravine in a harsh winter. Perhaps when the trees were smaller, but strong enough, to support this weight the snow was very deep and the boulders roiled into the right spot. Summer comes and viola a rock in the tree.
Yep, that will do it. I own a chunk of aspen forest in the Nevada mountains. For a couple years both I and my friends could not make heads or tails of some of the really odd things that were in evidence in that forest. Things way out of place and with no good explanation of how they got there. Pieces of tree in the tops of other trees, healthy trees snapped off halfway up the trunk. Big rocks in places we couldn't explain.
One year I went up to the property with some friends in early spring, when there was still some snow on the ground. In the valley, the snow never becomes more than a few feet deep, so it never occurred to us that avalances were possible. And then we saw a huge avalanche barrel down the mountain-side, taking huge boulders with it and crashing into the forest. It suddenly explained everything that had puzzled us over the years.
We actually climbed to the top of the mountain that the property backs onto that day (safe, since only the north-facing slope still had a significant snowpack) and discovered that the snowpack was still around 20 feet deep up near the top. We did not know that it EVER got that deep, but apparently that is a normal snowpack for that time of year. Not bad for a piece of Nevada desert, and it explains why the aquifers are always full down below where there is only a few inches of water precipitation a year.
Back to the topic, an avalanche will definitely do it, and you can find avalanche damage in places where you normally wouldn't think to look for it. In our case, only the tops of the mountains had serious snowpack, but we only had mild precipitation further down. The heavy snowpack at the top of the mountain would come barrelling down and do damage that otherwise would be inexplicable after the snow melted because there was little evidence down below that avalanches of that size could occur.