Tree roots could well be the culprit. If so, it sounds like the trees have also provided enough shade to protect the exposed rhizomes from sunburn.
In a message dated 9/4/2010 12:26:07 PM Mountain Daylight Time, email@example.com writes:
The soil is heavy, but filled with stones and gravel here. What about those tree roots? Could they be pushing the stones up causing upheaval of the rhizome. Hasn't happened under other trees before, but I'm not sure those areas had quite the stone content of this spot either. Lots of chunk sandstone came out under these trees. Also, it's the highest part of a long bed and I expect some erosion by both wind and water is at work. Nothing much is ever level here. It's been wetter here this year compared to the last few. Or at least the rains were spread out rather than coming all at once. I think the rainfall is still less than average. It just had good spacing for the vegetation.
I would leave them alone, but too many are stand alone plants with no or only one or two siblings due to the nature of the cross. I may never be able to get a repeat of the cross.