1. To cause to fall or collapse.
2. To collapse due to fatigue, an illness, or a sudden attack; SYN. collapse.
3. To fail to operate correctly; to break.
4. To lose control of one's emotions; SYN. lose it.
5. To make ineffective; SYN. crush.
6. When something decomposes or reduces to its smallest parts or is reduced by someone to its smallest parts, it breaks down.

When you reduce a process, situation, problem, plan, or idea to its basic parts to make it easier to understand, you break it down.
When you use force to go through a door that is locked, you break it down.