Maple Syrup begins to crystallize when you heat the maple syrup to a temperature of 100 F or above.
The higher the temperature the maple syrup is heated too the more it crystallizes.
In Celsius that 100 F is 38 C in which the maple syrup crystallizes.
If you don't want the maple syrup to crystallize then you need to keep the maple syrup cooler than 100 F.
I've had bottles of maple syrup that were in my car left out in the car after shopping that I forgot to bring in and the temperature inside the car reached 165 F with the windows rolled up and the outside temperature was 110 F.
The maple syrup when I went to get it had crystallized.