What event do you think caused the universe to start expanding?

Well personally, I think it all started in the beginning, when g-d created the heaven and the Earth.  You see, the Earth was void and chaotic, and there was nothing but darkness, and the spirit of g-d moved across the depth of everything.  At one point, g-d said "Be light" and there was light.  G-d was satisfied with the light, and he separated the light from the darkness.  

Of course, there wasn't any "mass" yet ... not even elementary 'subatomic' particles.  So, in order to allow the light to separate from the darkness, the density had to decrease enough to allow the energy to decouple from the primordial mass/energy medium.  And we all know that the only way to drastically reduce the density of everything you've got is to rapidly expand it.  Sure enough, modern Cosmology sees evidence of a huge exponential inflation beginning around 10⁻³⁶ second after the 'big bang' singularity.  

In my opinion, THAT's how the expansion began.

It all goes together, and modern Western religion and science are in complete agreement ... at least for 10⁻³⁶ seconds after Creation.

b. a lightning bug, plug of grass, and a worm