What the .......?????

[luckyphil]

How did this happen? What caused it? All 4 tyres (15" Toyo Open Country A/T) are treated with respect, inflated, rotated, re-aligned, balanced etc as needed. But this was found this morning, on the left rear inner. Photo doesn't really capture the damage which is limited to the inner extremity of tread and wall, perhaps 1.5 - 2.0 cm wide.

Could be rear trailing arm out of alignment or perhaps rear wheel bearing