Why are scissors starting to displace swords as the cool weapon of choice for main heroines to wield? (Inu to Hasami wa Tsukaiyou, Kill la Kill)
Not necessarily a trend, perhaps, but it's worth noting that there's more than enough anime with swords (http://tvtropes.org/pmwiki/pmwiki.php/Main/KatanasAreJustBetter), even guns, that when a unique, less-frequent choice of weaponry comes around, it may catch the attention of a frequent anime viewer.
Example: Would Bakemonogatari episode 1 would have been more interesting if Senjougahara used a sword instead of a stapler? Probably not, unless you had some philia regarding swords.
It is part of the clothes theme in Kill la Kill (even the school building is a school uniform!). So the scissors alone are not following some trend.