Then I would have to go with either the line pressure was set too high or your transmission was too hot while driving sometime. One of the biggest killers of automatic transmissions is heat. A lot of people disregard this issue and end up with blown transmissions. Its not just the power that is being put through it. If you got a standard transmission, you might be looking at a dead 10-bolt rear then. That's one problem with the 4th gens. With the A4, you're on a ticking time bomb sometimes with the transmission. With the M6 you're on a ticking time bomb regarding the rear differential.
Also, did you buy this car new? If you bought it used the transmission could have already been abused and was just waiting to go. Another possibility is that someone put in a higher stall converter with too high of an STR rating. Higher stall converters also add a ton of heat to the transmission. Many people put in higher stall converters without adding a transcooler. The 4L60E really does need a decent transcooler stock. Another thing, the 4L60E is not very strong with W.O.T. runs while in OD/4th. When I race or go driving hard I'll leave it in 3rd and reserve OD for cruising.