Hickory Dickery Dock. 
The mouse ran up the clock.  
The clock struck one.  
The mouse ran down.  
Hickory Dickory Dock.

Question:  Why did the mouse run up the clock?  

Nursery rhymes form part of English literature, and often contain hidden meanings.  To answer the question, we should search for other clues in literature.

The opening line of “1984” by George Orwell (1903—1950) recites:  “it was a bright cold day in [October] and the clocks were striking thirteen.”  Isn’t that one o’clock?  Orwell also wrote “Animal Farm,” which featured animals as the characters, presumably including mice.  It does not make a lot of sense, but Orwell did not make much sense either with some of his expressions:  ‘War is peace,” Freedom is slavery.”  ‘ignorance is strength.”  Obviously, sometimes we need to make sense out of nonsense.  Just imagine how confused that mouse must have been.  

Perhaps another Literature clue may come from “Alice in Wonderland” by Lewis Carroll (1832—1898) when the white rabbit was running through the field, consulting a big pocket watch from his waistcoat, exclaiming:  “ I’m late!  I’m late! I’m late for an important date.”  Could it have been a date with a mouse at one o’clock?  Maybe, but on the other hand, the white rabbit could have been just as mad as the Hatter.

James Whitcomb Riley (1849—1916) wrote:  “When the frost is on the punkin and the fodder’s in the shock, O it sets my heart a-clickin’ like the tickin’ of a clock.”  Except for the “gobble” of the turkey-cock, “clackin’ of the guineys,” and the “clackin of the hens,” we find no evidence of mice, but they must have been somewhere near the fodder while the clock was tickin.’  No help here. 

Eugene Field (1850—1895) in his poem “The Duel” wrote: “T’was half past twelve”, when the “Old Dutch Clock … up with its hands before its face” witnessed a duel between the gingham dog and the calico cat.  Note the time — a few minutes before one o’clock, but I expect any self respecting mouse would have been in hiding with the calico cat lurking around.  

After all this clock tickin’ analysis, we can only speculate on the answer.  The truth be known:  no one knows nor cares why that mouse ran up the clock.