Why did my Xbox One download Apex Legends on its own?

Are you positive no other account has your Xbox as their home Xbox? This situation sounds like that is a possibility.

Were you checking out the game at any point on your console or the Xbox app? Could be an accidental click, I know I've done that before. Do you share your console with anyone else?

If you're concerned someone may have accessed your profile, make sure you change your password.

Hi there NYC,

Thanks for reaching out here in Xbox Forums. Unfortunately in reference to your issue, the only way to confirm a download you will need to physically click on the "Get / Buy" option. This then places a transaction line on your Microsoft account.
If you purchased/downloaded the content on your account, please take a moment to
view your Xbox purchase history online
to confirm the purchase.

If the issue persists, I'd recommend updating your Microsoft Account password for safety.

Hope this helps.

You might want to update your account credentials just to be safe.

Who's to say it's not being auto downloaded on all consoles? I mean it is free, so it's not that big of a deal, but I can understand the frustration of taking up internet usage and storage space. I doubt that this is the case, but I figured I'd state the
other obvious option.