10. To scare off the Borg.
9. Chronition particles caused massive hair follicle damage.
8. Out of boredom, after Data read to her his new poem.
7. She ran out of wig attachments.
6. Because it was a good day to dye.
5. She caught the genetic hair disorder; Pamelaris Andersoritis
4. After a dimensional jump aboard Starbug, Beverly took fashion tips from The Cat.
3. Beverly used a 300-year old bottle of Vidal Sasson to do her hair.
2. As a dare, after long hard nights of tequila drinking with Troi.
And the number one reason Beverly's hair changed in 'First Contact'
1. She wanted to steal Pulaski's title for Worst Hairdo in Starfleet.