There are a lot of rumours flying around right now (although Joe Mallozzi has promised to make a blog post about it today) but most of them seem to come down to money.
After the success of the two SG-1 movies MGM has realized they can make good money through the movies so are moving in that direction for the show.
When SG-1 ended they came up with an idea for a new spin off series called Stargate Universe. It can be cheaper to produce new shows since they already have sets and new actors cost less.
MGM is thinking of starting it's own TV channel and if they do they can air SGU on there rather than through Sci Fi (which holds the airing rights for SGA) and not have to share the profit.

I'm just... so angry. I don't know if it's going to help at all, but I'm writing letters right now. Maybe it'll help. I'm also keeping an eye on reactions. I'm sure within a few days someone will come forward with a well organized Save SGA campaign and I'll probably take part then. I'll try and keep the group updated with what I find.