It might be self perpetuating now but I'm pretty sure the monkeys were semi-trained to do this by unscrupulous Balinese.
I thought I was being hyper aware, but the Monkey that got me was more cunning than I was. He'd hid behind a pillar and when I turned around jumped on my head and tore the glasses off my face. He then proceeded to sit on the edge of the cliff chewing on the frame while I watched on.
All of a sudden a group of kids descended on the scene throwing fruit at the monkey. The monkey picked up the fruit, dropped the glasses and one of the kids jumped the fence to pick them up. An old Balinese, Fagin like, guy appeared and suggested I should give the kids some money as a reward and I handed over Rp. 50.000. About 30 seconds later I realised I'd been conned and left never to return. Sad!