Could this be a sign that someone else has access to my account or can this randomly happen? it doesn't happen frequently, just every now and then.
Invalid tokens are caused for a few reasons:
Someone logged in to the account from a different location or at a different time, creating a new token and thus invalidating the previous ones.
You copied either the
launcher_profiles.jsonfile or simply your entire
.minecraftfolder to another computer, which will of course create an invalid token as both computers will be trying to use the same token.
It could be caused by using various third party launchers, as these require separate tokens to log in.
Other causes include multiple Minecraft instances running or the token just timing out. The token timing out seems unlikely based on what you said. An unlikely cause is an incorrect system time, but I highly doubt that is the problem.
However, as it doesn't happen all the time, it could simply be a fault in the operating system or your Java/launcher version.
That's all the causes I could find. It is most likely just an operating system or software problem.