Merriam-Webster online offers:
from the phrase kick the bucket (to die)
First Known Use: 2006
Scripted by Justin Zackham, “The Bucket List” refers to a wish list that two terminally ill men try to fulfill before each kicks the bucket. After they break out of a cancer ward, they head off on a road trip with an itinerary that includes racing cars, eating giant plates of caviar and slinging poker chips in Monte Carlo.
—“‘Bucket’ Brigade,” Daily Variety, June 29, 2006
It is also an earlier computing term:
High performance networking, IV: proceedings of the IFIP TC6/WG6.4 Fourth International Conference on High Performance Networking, Liège, Belgium, 14-18 December, 1992, Volume 4, A. Danthine, Otto Spaniol, North-Holland, 1993, pp. 340, 341, 344
[search inside the book for “bucket list"]
I thought it gained legs as a term from the movie of the same name.
I’d like to hear that it came into use in that sense as a result of some computer guys sitting around, joking.