Well, only one of two things; a double, w/ BE, likely, or a squib followed by a full pwoer load. I would assume you would have known if there was a squib event & checked. My bet is double. Just another reason I like high loft powders that fill the case at least 1/2 full.
if someone gets a double charge on a automatic index press, it's oprator error.
Well, anytime you get a double, it's operator error, regradless of press type. As always, be a handloader not a handle puller. Verify EVERY charge visually. If you can't tell the charge is a double or single, change powders, life is far too short.