Guess: Logged in using Chrome? IIRC Chrome has specific functionality regarding retaining and linking Google account information. This is how you get the account-selector.

As mentioned in a sibling comment: Utilize browser profiles (Chromium-based/Firefox) or Multi-Account Containers (Firefox).

This may very well be the case, but it was years ago so I don't really know. This type of linking is something that always gives me the willies when it happens without warning and makes me think I should stay away from using components that are too "smart." I got really freaked out once when I was using a portable wireless access point owned by my employer (for on-call duties) in a starbucks and it told me a second device had attached to it. Turns out I had just bought a new iphone and Apple had stashed away the wifi password and automatically given it to the new phone, rather than the explanation being some hacker sitting near me in starbucks. I completely understand why they did this, but it really would have been nice to be asked if I mind if they store a copy of my password in the cloud.