This pile of colluvium or angular rocks is called talus. Talus forms when rocks fall or topple downhill.  Construction on Tempe Butte broke apart rock that was dumped over the side.  Gravity took over; the talus rolled and slid to a stop. The general term for downslope movement of rock material is called mass wasting. 

Most of the talus you will see in your own personal travels will be from natural rock falls.  However, many slopes on Tempe Butte are littered with talus made from human disturbance.  For example, making the "A" on Tempe Butte generated fresh talus.