Locked phone

Hi. I've got a concern about webroot on my android phone.
 
This evening while I was making a call, my phone came up with the "device locked" message. This is despite the fact that I had not activated any of the lost device protection actions, changed my sim card or done anything that should have resulted in my device being locked. I also noticed that the "emergency call" feature of the locked device screen didn't work - pressing it just prompted me to enter my password again, so had I needed to make an emergency call during this time I would have been unable to, which is a bit worrying.
 
I have since managed to unlock it by resetting my password online, but I am concerned about how it came to be locked in the first place. I use this phone for work, so if this were to happen again during the work day and prevent me from making or answering calls it would be a big problem for me.
 
Would anyone be able to let me know why my device might have been locked? Prior to this happening, I had only been using the phone to make normal calls, to numbers which I call frequently.
 
Thanks in advance!