Just over the event horizon are strong vacuum fluctuations caused by the strong gravitational field around the black hole, those fluctuation generate particule pairs

for pairs generated at the edge of the event horizon for example an electron positron pair one particule will fall back into the black hole but its anti particule will escape because particule and the anti particule are pointing to opposite directions

The energy for this is supplied by the black hole gravitational potential

this result in bh mass loss

Edit, out of curiosity why downvotes?