Call it an abundance of caution – Spotify got hacked recently, resulting in one user’s information getting compromised. That didn’t include password or payment information, and there’s no danger suspected for anyone else, but Spotify isn’t messing around anyway.
Spotify will be prompting certain users over the next few weeks to re-enter their log-in information. The biggest risk seems to be for users of the Android app, which will be receiving a mandatory update in the next few weeks. Basically, Spotify will yank support for the current version of the Android app, necessitating a download of the new update, which will presumably fix whatever security flaw was detected.
So, it sounds like nothing to worry about, but hey, better safe than sorry.