Causes Of Lost Circulation in Drilling
In general there are two main causes of lost circulation in drilling operations problem which are as following:

- Invasion or mud loss to formations such as cavernous, vugular, fractured formations or unconsolidated formations. 
- Fracturing which is mud loss due to hydraulic fracturing operations from excessive induced pressures.
https://www.drillingmanual.com/lost-circulations-causes-types-drilling/