Broken clutch cover bolts

Only time I've ever broken cover bolts was on a M1000. I discovered that the shoulders on the bolts were too tight in the holes in the cover on a few of them. So when the bolts were being tightened the shoulder would bind and start stretching the bolts in the threads to try to pull the bolt through the cover (yes I was tightening the cover evenly and not letting it get crooked). Ran one size bigger drill bit through the cover bolt holes and fixed the problem.

Usually a symptom of something warped or binding in the tower or cover. Check for flatness with the cover and spider and check for cracks. Look at bushings and look for binding or excessive movement. As stated here, look at binding of bolts. Did you by chance replace them or have them out and reinstall? They are very easy to over torque and break or weaken...less is more. Use blue loctite to assure smooth torque down with no binding. Only time I've broken them was due to cracked spider on M1000 that I didn't see/catch...spider broke and the issue became obvious.