if there is no rhyme or reason why something happens, there is no obvious explanation for it.

I don't know what makes her behave like that. There's no rhyme or reason to it.

• dive into something
• divide-and-conquer
• divide and conquer
• divide and rule
• divide-and-rule
• do a double take
• do a favor
• do a favour
• do a job on
• do a job on someone
• do a job on something
• do a land-office business
• do all the running
• do a mischief
• do a moonlight flit
• do a number on
• do a number on someone
• do a number on something
• do any favours
• do anything by halves
• do a power of good
• do a roaring trade
• do a slow burn
• do away with
• do away with someone