I'm not sure how that happened because on any angle grinder that I have used the direction of rotation of the blade tends to tighten the nut - sometimes to the point that it is v. difficult to undo. Also the guard (assuming it was still in place) would tend to hold the blade and disrupt any spin should it come loose. The blade certainly looks to be spinning at full revs. and a narrow escape to be sure.

As PIH states the rotation of the blade should tighten the nut. My bench saw has a LH thread and if I was to stall the blade for any reason, then the bolt on the arbour would tighten. My guess is that this blade was not in a proper machine as it would have been encased and unable to escape.