There are many nooks and crannies in a modern dslr mirror box to hold (and hide) dust. If it's just dust - not fluid - I use a strong blower (like the VisibleDust Hurricane Blower) with the body held lens mount down. As I compress the blower with a quick, strong squeeze, I quickly (but carefully) move the body up, away from the blower in as smooth a motion as I can.

This usually gets rid of the larger bits, but it also can send the dust into other spaces within the mirror box, so it usually takes a few hits before I'm satisfied. Make sure the blower isn't too far into the mirror box that it comes in contact with the sensor.

I also do this with my lenses, mount-side down, cause there are lots of nooks and crannies in them as well.

Static is the primary culprit - every dslr sensor is electrostatically charged. Even though most bodies have an anti-static coating, there is always static present. Dust can adhere any time the mirror box is open to the air. In my experience, lens type has no impact. Once the mirror starts slapping around during capture, it all gets rattled loose, and goes straight for the sensor.

If the material on the sensor is fluid-based, there are other tools available - a blower is basically useless - but the process for remoal is much more daunting, involving contact with the sensor. As such, I recommend seeking professional assistance.