Happened to me with an LG-V10 H960 (Israeli model, 32GB) - about an hour ago. This is with a plain, non-rooted phone, with stock ROM that only upgraded to Marshmallow a few months ago. Never fiddled with it, nothing at all.
I noticed over the past few months an occasional case when the phone would tell me I need to tap my knock-code and can't use the fingerprint reader, so I started suspecting it may have rebooted spontaneously when I wasn't looking, but I couldn't be sure as it kept saying "after 48 hours of no use - cannot use fingerprints" - it seemed odd, but I didn't read the signs...
About an hour ago, I was holding the phone, and it happened right in front of me. Phone became stuck, unresponsive for a few seconds, then screen blacked-out, and I saw the LG logo, and it was stuck in the bootloop.
I pulled the battery, and replaced it, tried a few times - nothing. Then I pulled the battery again and tried to only charge the phone - it showed me the charging screen, but then rebooted and went back to the bootloop.
I'm on my way to an LG service center, and will update if they tell me anything interesting. In any case I got "lucky" since I ran LG Backup about a month ago, and all of my photos / videos get recorded to the external SD card (which I immediately removed, of course, for safety).