I've explained that. Because the source of energy providing mass for the singularity is finite, eventually, the singularity will destabilise, collapse and expand. Eventually, the gravitational pull of the centre will overcome this, and we will once again collapse.
I think you may have a couple of misconceptions there.
Firstly (as I understand it), there is no "centre" to the universe, just as there is no centre to the surface of a balloon. As it inflates, all points move away from their neighbours - there is no one point (on the balloon / in the universe) which is the source. Gravity acts rather like the tension in the rubber of the balloon - if you blow too hard, the balloon may burst - if the air pressure stabilises, a fixed size may result. Only if the pressure drops off will the balloon collapse.
The nature of the inflationary force is not well understood. The early universe expanded very rapidly, and then the rate of expansion diminished. The most recent reports I have read suggest that inflation is slowly increasing, and we are eventually in for a "big rip", rather than a "big crunch".
The "finite energy" argument is not definitive. It takes a finite amount of energy to start a ball rolling downhill. That does not prove that it will roll back up again. The inflation I referred to above appears to act rather like an energy gradient, analogous to "downhill".