two potential causes:
1. water is contacting the bulb while it's at full burn.
2. the bulb was handled with bare hands/had some other source of oils touch it
Either way as Ponky said, that's a whole lot of toxic material raining down on the plants, not to mention the fire risk. I would reconsider using HID if this has happened more than once. These aren't like your regular incandescent or CFL light bulbs you put in your house. They are hyper sensitive to oils on their surface and cannot have any water touch them when at full burn. Technically, if this actually happened, one is supposed to call a hazmat clean up crew to deal with the situation since it's super toxic and the standard homeowner/renter/person simply doesn't have the skills or resources to properly clean up the mess.