Err: FAILSAFE_EKFINAV-1 due to NSats going 0

I am just trying to understand the cause of EKF FAILSAFE triggering. In my case NSats shows zero value for sometime which caused EKF Failsafe. So I wanted to know that is it possible for NSats going zero for sometime or it happened due to some hardware issues as it had happened few times earlier also.
Please find the log here