They only have a ceiling of about 10,000 feet and a range of a few miles. For SA7s or any of the other man-portable SAMS to be the cause, the planes would've had to go down very soon after takeoff. That wasn't the case. From what I've seen, they were at cruising altitude. It'd take a much larger SAM to hit them there, not exactly the kind of thing that's easy for terrorists to get their hands on and use.

There are many possibilities if terrorism proves to be the cause, including bombs and hijacking attempts gone wrong. It's really too soon to speculate much beyond that. Unless some cause is very prominent, it could take weeks or even months to determine the cause(s).