Could be any number of things... but the ultimate responsibility for the safety of the aircraft and its passengers is placed on the captain. An aborted takeoff call could be initiated by ATC in the tower or by the pilot flying the aircraft.

During the takeoff roll there are four calls made in the cockpit:

"80 knots" -- this is the point where nosewheel steering is disconnected and the aerodynamic surfaces begin to have effect.

"V-1" -- Before this call is made the pilot can abort a takeoff (this is usually around 145-150 knots, and it varies by aircraft weight, field altitude and the temperature that day). Beyond this call, the pilot is committed to takeoff.

"Rotate" -- this is when the nose is raised and the aircraft lifts off.

"V-2" -- Usually about 10 knots or so above rotate speed; at this point the aircraft must be at a positive rate of climb as indicated by the instruments. The "gear up" call is followed immediately afterward.