Casing failure

Anyone have a similar experience or possible explanation for the following:  Trying to avoid leaving this an oilfield mystery.

13â…œ"68# N-80 BTC casing ran on semisub to 4350' in vertical hole section.  Landed out and cemented with no issues.  Bumped plug and began pressuring to planned casing test of 3500 psi.  At 2100 psi the string jumped and pressure went to 0 psi.  POH and found pin on 15”² pup joint below SS hanger had released from BTC coupling on 1st full joint of casing. 

1. Several observers confirmed connection was made up within 1/16" of base of triangle with 12,000 ft lbs torque.  Weatherford Lubeseal thread compound used.
2. Calculated tensile load on connection at time of failure was 475 kips.  Conn rating is 1586 kips.
3. Dimensional check of pin and coupling were within API spec (Both pin and coupling were recovered and sent in for inspection)
4. Multiple calibrated gauges confirmed pressure did not exceed 2100 psi.  Tong torque gages were calibrated.
5. Pin and coupling had minimal thread damage.  No stripped threads.
6. Pup joint was cut from joint of same batch of casing run in well.  Inspection after manufacture within API spec.
7. Plan to send pin and coupling for failure analysis.

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