Eh it's not that unusual with MMOs to see people leave during lulls in content patches or when other big MMOs come out. Many come back later when new stuff is released. GW2 made some people leave for a while but they got bored with that and came back.
I'm not on the GAF server but my guild is still really busy and raiding throughout the week.
Oh yeah, I know, it's just a shame because SWTOR ends up with so many big lulls espcially if you only play one side of the game. A year ago, it was the PvP players yearning for content after being shafted for who knows how long and now it feels like PvE players haven't gotten anything at all since DF/DP launch and NiM is nice but that really only draws the hardcore crowd (which I'd love to be but no group)
I've just been noticing it more because I see big guilds on my server slowly disappearing as well.