Fear, speculation, and hoarding which were sparked by the legislative (non, as it turned out)flurry to "do something"--generally, to add more restrictions to everything. It never happened nationwide (but NY passed this odious S.A.F.E. Act. Background checks to buy ammunition! 10-round limit on mags, but only permissible to load 7 rounds unless at a competition or a range. Broader definition of "assault rifle", to include MANY more models, etc, etc.
) I have seen no evidence that production rates were manipulated, but this also is suspected.