Bill,
The cause behind every other Ta-152 loss is known except that single one. And it absolutely couldn't have been caused by enemy action as both Willy and his wingman saw him just dive out of formation and into the woods, no tracers, smoke, enemy a/c or damage to Stattler's a/c was observed.
So that leaves only a few possible explanations, either he fell asleep, got a heart attack (rare yes, but when have you ever heard of this before besides this ??) or mechanical malfunction (Highly unlikely as he didnt respond).