Hi Len,

I'm sorry to hear about your tree.

From what I can see in the photo, that is almost certainly the work of humans--the cleanness of the cut is telling.

The other remote possibility is that it is coincidentally associated with ash twig beetles, which tend to leave exit holes in straight lines that can, in some sense, perforate the bark and make it easy to remove along a straight line.  

I hope this is helpful.

Cordially,

John