High humidity in that area and then at altitude they froze over - however - checklist calls out pitot heat ON. There are other means to check what’s going on with airspeed indications and how to over ride.
Rumor has it the Co-Pilot did climb out and try to pee into the tube to melt the ice, but couldn’t because the cover was left on. He then came back inside the plane to ask the pilot if he should take the cover off. The pilot said yes, then the plane crashed.