Several flight instruments rely on air pressure. If one of your pressure gauges gets blocked, you can get very misleading data.

This guy's theory as to what happened in this case is that the forward pressure tube (pitot tube) had an ice blockage, which led to a low airspeed reading. The autopilot tried to correct by accelerating the plane. Since loaded jetliners normally fly slightly nose-up to maintain altitude [1], the autopilot kept that orientation as it accelerated, which led to a climb and ultimately a stall.

When the plane stalled, it started dropping altitude rapidly -- but with a blocked pressure line, this could misleadingly report a high airspeed due to the increase in external pressure. As the pilots took over, they were (according to this guy's theory) trying to reduce airspeed by cutting the throttle and nosing up. Rather than fixing an overspeed condition, they were actually making the stall worse. They never would have realized what was happening because their instruments kept reporting overspeed.

[0] I'm not a pilot, but I worked in an aerospace museum's education department, and learned a lot from the pilots and engineers who volunteered with us.

[1] According to one of the volunteers, who was also an engineer for one of the big jetliners, loaded jetliners get about half of their lift by keeping their wings 3-6 degrees above level. The exact angle is selected based on load and airspeed; get it wrong, and you'll accidentally climb or descend.