I replaced the timing belt about 800 mi ago (I have done this more than a dozen times over the years for various volvos). My wife called me from the freeway - When I got there, I saw that the timing belt had shifted towards the front of the engine, cut a hole in the upper and lower timing belt covers and about 1/2 the width of the belt had shreaded (the remaining part of the belt was still intact, but it had skipped several teeth- the belt did not break). I have never seen this type of belt failure. What are likely causes of this (could I have caused the failure by not re-tensioning the belt tensioner after 300-500 miles)? The previous belt had been in place for about 70,000 mi (it had not obviously worn, but I was replacing it because it was that time).

thanks