I think it is a combination of factors.
1. rumors abound. Anything from banning of "military calibers" to "bullet taxes". What would a military caliber be? When I was in, I used 22lr, 223, 45, 9, 38 and 308 (yeah, yeah, I know, I know). If a 50% "bullet tax" was added, then you have made that up in equity. If you can no longer order thru the mail (didn't we go thru that in 86?), then you would be at the mercy of local supply.
2. Deja Moo. sort of like deja vu, but instead it means "you've seen this bu!!sh!t before". I went thru it in the 80s and 90s with a Bush ban and then the Clinton years. You couldn't find any ammo during the Clinton fiasco. And then when BO got elected the first time, you couldn't find anything. So people learned. And panicked.
3. I think I can make a quick buck. If you buy out a local supply, they you get to dictate the price. Look at the prices at the last gunshows. 223 for a buck a round!!!
4. You look on the classifieds and armslist and see everyone posting a WTB Ammo, or WTS ammo at highly inflated prices and you think, I better get mine while I can.
5. you replay all those idiot voices in your head that say "he's not going to take your guns" and then hear "If I am re-elected, and Congress sends me an updated Assault Weapons Ban, I will sign it" followed by "The President can take Executive Action on this matter."
That kinda stuff.