If, for example, a problem develops with the air regulator, the restricted air flow could prompt the diver to ascend rapidly enough to cause an air embolism (bubble) in the bloodstream, which can be fatal. This would be considered a panic response if the diver had access to a pony bottle (an emergency air supply), or other divers who could share their air supply, allowing a gradual ascent.
There have also been eye-witness accounts of divers removing their regulators underwater, apparently because they incorrectly thought their air supply was inadequate.
