Has it happened before?
Do you run on carpet?
I used to run a T2 a few years ago on carpet and suffered this on occasion. Once a belt has twisted once it is wise to change both of them as their structure breaks down and become a lot more likely to repeatedly twist. I found the main cause to be debris from the carpet (compacted fluff) mixed with tape adhesive and tyre additive. The diff pulleys are not too hard to check in situ but the drive pulleys really need to come out first.
Once you have the belts off, check every pulley for compacted stuff and scrape it out with a small watch screwdriver.
Hope this helps.
