By means of numerical simulation, the last stage of the propagation of a premixed flame in a closed tube is investigated. It is shown that the flame front displays a cellular structure that has a close connection with the acoustic waves. The following scenario is derived: first, pressure oscillations are triggered as the flame reaches the lateral walls and suddenly decelerates. Then, a slender cusp appears that further collapses; during this stage, acoustic waves are only superimposed to the flame front and the interaction is weak. After the collapse of the primary cusp, the periodic acceleration drives the flame to display a cellular pattern oscillating with a time period that is twice the acoustic one. This process progressively leads the total heat release to oscillate in phase with pressure to produce a violent instability.