My guess is that the rubber dried out due to sitting for so long. Was the bike stored in a garage that gets really hot?
Chances are they just dried out. What's wierd is the front ones didn't.
Do you use more back brake when you ride?
If so, consider changing your technique. You get more stopping power and control while stopping if you use more front brake. If you are skidding your back tire, you are not in contol. Think anti-lock brakes.
You should use about 70% front brake/30% rear.
Next time you ride start with 50/50 and then increase your front as you go. If you lean back while applying your brake you WON'T flip over the bars.
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger