Hoodoos in the Bryce Canyon area are formed by two weathering
processes that continuously work together in eroding the edges of the
Paunsaugunt Plateau. The primary weathering force at Bryce Canyon is
frost wedging where there are over 200 freeze/thaw cycles each year.
In the winter, melting snow, in the form of water, seeps into the
cracks and freezes at night. When water freezes it expands by almost
10%, gradually prying open cracks, making them ever wider.
In addition to frost wedging, what little rain falls in the area also sculpts the hoodoos. Although the Bryce Canyon region is far from major sources of atmospheric pollution, rainfall there is nevertheless slightly acidic. This weak carbonic acid can slowly dissolve limestone grain by grain. It is this process that rounds the edges of hoodoos and gives them their lumpy and bulging profiles. Where internal mudstone and siltstone layers interrupt the limestone, the rock will be more resistant to the chemical weathering because of the comparative lack of limestone. Many of the more durable hoodoos are capped with a type of magnesium-rich limestone called dolomite. Dolomite, being fortified by the mineral magnesium, dissolves at a much slower rate, and consequently protects the weaker limestone underneath it in the same way a construction worker is protected by his/her hardhat.
Rain is also the chief source of erosion (the actual removal of the debris). In the summer, monsoon type rainstorms travel through the Bryce Canyon region bringing short duration high intensity rain. --from Wikipedia