Something like that, actually. Scientists have suggested that the universe repeats cycles of contracting and exploding and we're currently at an expansion portion following an explosion. Any time energy is expended the whole system loses that energy (unless it has a mechanism to recapture 100% of that energy). So, unless there is some mechanism to insure 100% of all energy involved in the universe is recaptured at the posited collapse of the universe then eventually it will not have sufficient energy/mass for the explosion. Remember, atheists must claim something has always existed, and their choice is matter/energy/existence. Given an infinite number of these cycles of contraction and explosion eventually it would run down if the system loses even just the most infinitesimal amount on each cycle...remember we're talking eternity here, so infinite repeating of this process.
Also what does "prime mover" mean? Is it something like when I throw a baseball and it leaves my hand the prime mover is there to push the ball through the air?
No, what I meant by that term was that even if one were to assume existence just "was", the universe is in motion, it's not just static. You may recall the law of science that says things at rest tend to remain at rest, and things in motion tend to remain in motion unless some other force is applied. So what began the motion of the universe? Physical laws would expect motionless existence to remain motionless...so there must have been some prime cause, which I was just referring to as prime mover. Is that clearer?
Codswallop. The "big crunch" does not have the evidence to support it. There never was a "before" the Big Bang since time itself started with the Big Bang.