Basically, Democans and Republicrats both decided to spend an enormous amount of nonexistent money since the election is looming.
The bill sent to the White House had 34 pages missing. Bush either did not notice or did not care, and vetoed the bill anyway.
(Despite the "Farm bill" label, most of the expenses is said to be food stamps, school fruit etc. I wonder if it would be harder or easier to veto a Food Stamps bill.)