Silly question but here goes

[weecalf]

Why do relationships break down but marriages break up .Not why but why are they given different terms ?

[Canary42]

Because they travel on the bus but in the car :)

[joggerjayne]

For the same reason that

"filling out" a form

is the same as

"filling in" a form.

[joggerjayne]

When someone is shutting "down" their business ...

... it's called shutting "up" shop.

[Manwithnoname]

they may be regional variations which mean the same

[grasscarp]

I sometimes think about the fact that your car slows up or slows down, but it is the same thing.

[RATTER15]

You hang your coat up but in fact it hangs down!

[grasscarp]

there's loads of them - you do a "write up" about something, but what you are really doing is writing it down!