Well, there are a few things

Starting with in flight possibilities
*Engine failure (were you using an addon that does that)
*Icing - do you use REX or Active Sky or something that simulates ice?
*Did you have autothrottle on by accident and not know it? (I've done that)
*Did you have the speedbrake up by accident (done that too)

Other things - have you modified the plane at all? If so, how so?