Why do major earthquakes keep happening in Lombok recently?
Yes, it is quite possible to have a number of earthquakes over an extended period of time.

The phenomenon is actually quite common. Some famous sequences are: New Madrid in 1811-1812, Eastern California Shear Zone (1992-1999), Italy (2009-2016), New Zealand (2010-2011) etc. These are all areas where rates of seismicity are relatively low so a number of events in quick succession (as you saw in Lombok) is not due to pure chance. 

The idea of one earthquake triggering another on a neighboring or nearby faults is not new.