It's either a defective tire, or as mentioned above, poor wheel alignment; altho having the front wheel alignment skewed enough so that you aren't riding straight down the road can also be as much of an issue as having a skewed rear wheel!! :shocked:
Basically, if your Spyder wheels are not aligned properly so that the front wheels are evenly balanced straight ahead of the pushing drive wheel, then you are going to get uneven rear tire wear - it just depends on where & how much the alignment is out as to where the wear shows up most!! I'd guess you probably also have uneven tread wear on the front wheels, &/or the 'straight ahead' position of the handle bars is slightly skewed away from correct as well! :sour:
When can you get to see Squared Away (link on the front page) & get your Spyder 'Squared Away'?? Or when can you get your belt alignment done properly & a laser wheel alignment done too?? Sooner rather than later would be good, but there's still a fair chance that your rear tire at least is worn beyond easy recovery & will require replacement fairly soon if not now!

pps:
Good Luck! :thumbup: