Any idea what causes this?

Mine sheared off yesterday.  New rifle (a build not factory).  About 200 rounds through it.

I ordered a lost parts kit from Bushmaster that has 2 retaining pins and 2 retaining pin screws plus the take down pin and spring that the back plate keeps in place.  Brownells had individual parts available also.  Colt and DPMS.  

Any idea what causes this?  I have an H buffer in carbine stock.  Cycled reliably up to now.  Pin moved up and down normally to install/remove the buffer and buffer spring prior to breaking off.

Probably just a random occurance but thought I'd ask.

Thanks!

It happened to me about 5 years ago and I replaced mine without issue after 12k rounds so far.
Could be a bad part. No way to tell. If it happens again, I would suspect the hole drilled in the lower is in the wrong spot.

I usually shoot suppressed.  But I have an adjustible Syrac gas block.  I wonder if I should try a heavier buffer?  this was an H at 3.8 oz.

This was my thought

With the upper and lower put together, the buffer shouldn't be touching the tip of retaining pin at all

Correct, this will be the result that shows up on the face of the buffer....

Great picture.  Thanks.  The front face of buffer looks like new.  I took my rifle to my NFA dealer and he replaced the retaining plunger.  Tip of old plunger was underneath trigger assembly.  He thought that based on color of broken post it had not been heat treated properly.  I am concerned that as you close the upper onto the lower, the bolt carrier only makes contact with the buffer when it is nearly all the way closed.  My other rifles contact the buffer about 1/2 way down.  the castle nut was not torqued correctly either (too loose).  Fixed that.  Back to the range.