If track get's removed it maybe because the Deal with with the Songs Publishers has gone or failed, Making deals with Publishers can be hard espically if it's warner.
Some songs may come and go, it's nothing what spotify can do to stop it, If spotify did put the songs up without the publisher letting them, this would be it would be ileagel and Spotify could get shut down.
Please note Spotify has done well to get all the songs up of what there have now, I am sure the songs will be back up at somepoint again.
Or it could just be that there are chaging the songs to better quality songs which can sometimes take while.
Or it could be some other reason of which I can not thought up yet.