Have you ever noticed the patterns in words that always come together?
Take a look at these groups of phrases:
Why do the phrases in the first group sound right while the ones in the second group sound wrong?
It's as if you just instinctively know that flip flop is the correct word and flop flip just sounds borderline offensive to your native English ears. However, this instinctive knowing of which word sounds right is just one of the many unwritten rules of English that native speakers just know. It's called ablaut reduplication.
According to The Elements of Eloquence by Mark Forsyth, ablaut reduplication is when you repeat a word with an altered consonant;
Willy-nilly hiddgledy-piggledy hocus-pocus mumbo jumbo hubbub
and sometimes with an altered vowel:
Flimflam shillyshally tittle-tattle wishy washy zigzag
In words where you change the vowel, the order must go I, A, O. Oftentimes the first word has the I sound, and so the second word must have either the A sound of the O sound. And that's why words like tick-tock, mishmash, and tip top all sound right!
As for why this is the case, well, many linguists would love to discuss and debate that with each other. But perhaps for those of us who aren't too keen on understanding the technicalities , let's just say these words are simply the ones we're used to because they slide off the tongue so much easier!