What cause it to cut the timing belt

Improper installition... Belt was cut or nicked when it was installed and now it finally showed up!!!.. Process may take several miles and some time but that is usually what does it... Especially if no other reason is found... I assume that's been looked at already..(?) or you wouldn't be asking!!

OR the car has over 130,000 miles on it and coolant from the water pump and oil from the cam and crank seals leaking , finally caused the belt to SNAP ?

if the belt broke idler pulley is a good thing to check