The answer to this question is supposed to be known since the early 80's of the last century and the relevant model has been extended to explain the outbursts of low-mass X-ray binary transient systems (also known as microquasars). However, some recent and rather worrying observations seem to undermine the "standard" explanation of dwarf-nova outbursts. I will present and discuss this confrontation of model with observations and suggest a likely way out that favours theory.