Now I know why!

RUBBER STAMP WARNING!!!

A recent study indicated that rubber stamps and embossing powders give off certain pheromones that actually hypnotize women and cause them to purchase unreasonable amounts.
When stored in large quantities in enclosed spaces, the pheromones (from rubber) cause memory loss and induce the nesting syndrome (similar to the one squirrels have before the onset of winter i.e. storing food), therefore perpetuating their species.
Sound tests have also revealed that the rubber emits a very high pitched sound, heard only by a select few. A breed of women known as "Rubber Stampers". When played backwards on an LP, the sounds are heard as chants,"buy me, stamp me, emboss me."
Studies have also indicated that aliens have inhabited the earth, helping to spread the effects that these stamps have on the human population. It's also been experienced that these same pheromones cause a pathological need to hide these stamp purchases when taken home (or at least blend them into the existing stash), and when asked by a significant other if the stamp is new, the reply is: "I've had it for a while."

Many thanks to Catherine of www.cathcreates.com.au for a laugh.