There's been quite a bit of speculation as to why or how Amazon started suddenly filtering out sexually-themed material, particularly gay- and lesbian-themed books and items.
I generally subscribe to Occam's Razor and would bet it was a stupid policy move that backfired. Hackers? Not terribly likely - the sheer scope of modifying a huge, complex system in a precisely targeted manner would require almost complete and undetected pwn4ge over a long period of time. Glitch? Again, targeting such a precise (and politically controversial) target? Please.
But if I had to pull out my tinfoil hat, I would suppose that if they wanted to mollify a certain group of customers who can't stand to see certain types of product on their site they could do something like this, retract the move, and then (rightly) claim they can't do such filtering because of the backlash.