Definition from Wiktionary, the free dictionary
break down (plural break downs)
- Alternative form of breakdown.
- (intransitive) to fail, to cease to function
- I am afraid my computer will break down if I try to run it at too high a speed.
- (ergative, figuratively) to render or to become unstable due to stress, to collapse physically or mentally
- She is back to work now, after she broke down the other day.
- (ergative, figuratively) to render or to become weak and ineffective
- His authority and influence over his coordinates broke down gradually.
- 2012 June 2, Phil McNulty, “England 1-0 Belgium”, BBC Sport:
- Hodgson's approach may not illuminate proceedings in Poland and Ukraine but early evidence suggests they will be tough to break down.
- (ergative) (to cause) to decay, to decompose
- Leaves and grass will break down into compost faster if you keep them moist.
- (ergative, figuratively) to divide into parts to give more details, to provide a more indepth analysis of
- If you don't understand, ask him to break down the numbers for you.
- (ergative) to digest
- His stomach took a while to break down his food.
See also 
to become unstable, mentally or otherwise