This is my first ever grow, im 3 weeks in and my 250 watt mh bulb seems to have exploded on the inside leaving a massive bulge on the outer shell!!!

ive ordered a new one but can someone please tell me why this has happend i cleaned the bulb with a cloth before applying and have pretty much done everything by the book.

also, ive had to replace it with my 250 watt hps until the new one arrives so ive raised the reflector a few inches and now giving the plant 6 hours of darkness instead of 4. the bottom leaves have already gone almost completely yellow and other leaves are beginning to look crispy on the edges as well..

please help me?