Oscar is right, it is due in no small amount to the order that once fired military surplus brass be destroyed rather than sold to vendors to reload and sell as target ammo. Reason 1.
Several internet rumors have circulated about nationwide "ammunition accountability" which is basically regulations that register ammo to the user. Most saw this as gun registration by another name. Reason 2.
Other internet rumors circulated that Obama was planning to tax the hell out of ammo. Reason 3.
The media has been squaking about huge runs on guns and ammo at gun stores, causing people to feel that they should get in or be left behind. Reason 4.
Finally, the AWB has been widely rumored to be supported by Obama. The AWB does not forbid the posession of so-called "assault weapons," rather just the sale. Ergo most reasoned that shoudl the AWB be reinstated (with no sunset timeframe as rumored), they would never be able to buy certain items again, so everyone started going on buying binges (me included, but for different reasons). Reason 5.
I find it hard to believe that the ammo industry colluded and caused a run, I think it's more of a snowball effect than anything else.