I dunno ladynorwood. It's a common pattern in BrE, where you have a stress shift between noun and verb, think COMpress (noun) vs comPRESS (verb); REsearch (noun) vs reSEARCH (verb). This fits nicely into that. And it seems AmE speakers are just as confused about how it should be pronounced, so it's just one of those things I reckon. Although, going over and over, harASSment [charges] feels more natural than HArassment, so it's an odd bod.