Blown RMK 700 Motor. Why?

Hvae u owned that sled since new. It is possible that previous owner un hooked that speedo cable and then hooked it back up to sell it and that sled has lots more miles. I hope there is no one out there that would be careless enough to do that but it is very possible.

check the thrust washer on the crank for the rods ,I had one come apart ,part of it was sucked up the port and took out the piston ,its hard to find if only part of the washer breaks off

That is a pretty old sled to only have that many miles. I use 500 miles per season as a guideline. Any less than that is a "red flag" that either the sled spent all its time in storage (far worse than being used) or that someone ran the sled with the speedo disconnected. As far as the question about what might be wrong, I too have seen a broken thrust washer on a connecting rod show up in a relatively low-mile 700 motor about the same age. I've also seen rust (from storage) destroy con rods and bearings, causing failure abnormally soon. Don't run the motor until it can be torn down and checked over. Those cranks can be serviced (even though Polaris considers them non-rebuildable), so if it really is a bottom end problem, you may not be into it for big $$. Yes, I'm sure you'd rather not have to work on it, but if you have the work done, you'll know exactly what condition the motor is in. Normally, they do run for a lot of miles needing only normal maintenance.