Sounds like the amount of current being pushed through the pins caused them to heat up excessively which further increases resistance which further increases heat, repeat ad infinitum. Combine this with the clamping pressure of your heatsinks pressing the CPU and pins so that the pins are making contact with the plastic backing(like SF3D just mentioned) of the socket and voila, melted plastic socket with a side of roasted CPU. I'd say you need thicker pins in order to successfully transfer these amounts of current without causing this to happen. The pins shouldn't get that hot..