After reading all of the posts I think one of three things happened:

A too light powder charge - the OP said that he could not seat a bullet with a double charge. A round with too light a charge can detonate. Many reloading manuals mention this phenomenon. I could easily see this happening if a piece of material (like from the seal under the cap of the powder bottle) or an insect got into the powder hopper, charge bar or powder funnel in the powder die. Also if the cases are cycled in the reloader too vigorously between the charging station and the seating station powder can be tossed from the case leading to a low charge weight.

A bullet wasn't crimped properly and recoil set it back in the case being fired increasing the case pressure beyond safe levels.

Some cleaning media was stuck in the case before loading it. The media caused over pressure in the case.

If the round did in fact detonate it would explain the adjacent rounds going off too.