Frontal resorption is caused due to:
Frontal resorption (direct/periosteal/forward resorption)

With light continuous forces, the bone is resorbed directly by a frontal osteoclastic attack. 
The first step in orthodontic tooth movement is the activation of osteoclasts.
In frontal resorption, resorption is initiated from the PDL side of the alveolar bone. It usually takes place after 2 days.
Light continuous force
Heavy continuous force
Heavy interrupted force
Light interrupted force