Unexpected LAND mode after switch to RTL

Hello guys,
need an advice with my copter 3.4.6. When copter is away from home position being in STAB mode then switch to RTL sometimes it starts descent. In such moments I fall into panic and switch back to STAB mode and get height manually back.
When I looked in the logs I was really surprised - there appeared after RTL
or LOITER almost immediate LAND mode. I’m really confused because I even don’t have LAND mode preset in my flight modes. Could please somebody advice me why LAND mode switches on itself even if it is not set in my flight modes?
I put one log file from my flight.
2017-07-06 20-30-41.bin (2.8 MB)

You accidentally hit an EKF Variance Failsafe…twice.
Each Failsafe invoked RTL.

You might want to work on reducing your vibrations:

X and Y axis vibes should be no more than +3 & -3 while Z axis vibes should be no more than -5 & -15

That can be causing your EKF Variance

Thanks a lot for advice. I will try to reduce vibrations.