Why would GMail lock you out?
No, I know my passwords for all of my accounts.
There are a number of reasons and I cant claim to know all of them or even necessarily which ones trigger it at a specific event.
Some of the things that bother them are logging out of google on some devices that they are trying to track you on, trying to log in on other devices that they want to track you on, deleting cookies, trying to log on to the same device with more than one ID(even at different times), insisting on trying to associate all of my ids with a phone number, and some unknown array of other things.