Does anyone have any ideas on why trees/stumps break off at or just below the water line? I can understand trees rotting and breaking at the water line where the tree meets the air, but why three or four ft. below the water line?

There's more abrasion coming from the waves. Thats also why alot of floaters show up after a good rain/wind storm

Boats hitting them?

Lake levels change; just plane old erosion.

I've heard in the past (here I'm pretty sure) that trees well under water don't rot, presumably due to a lack of oxygen from what I remember.
The area in the top few feet of a lake is constantly churned by wave action, so constantly oxygenated in that layer probably contributes to slow steady rot there.
Or when the lake maintains a "low" level for awhile, that may also contribute to rot at that level, then it comes back up to "normal".
Lately been fishing Lewisville due to economy (near the house), not choice, there is a major "stump" field marked north of Stewarts Cove on AID maps (and others I'm sure, that's what I looked at yesterday)... take it seriously ,those stumps that were 6' under are now at the surface, we fished around them yesterday, and before this drop, never thought twice about blasting through that area in the past heading towards Little Elm, stopped quickly yesterday when I noticed the birds standing "on water".

Thank the birds. They are good for a lot of clues. Haha

Almost ran into a bunch a stumps on texoma a couple weeks ago that i never even knew were there. out at night and was lucky to catch a glimpse of the ouliers before it was too late.

According to fishing reports Texoma is about 8 ft low.

Unfortunately it still is. even with all the rain the watershed recieved