Explosive tire failure after tube change

I am trying to understand if I am doing something wrong.

I have had a tire wire bead pop off the rim with a loud KABOOM after
replacing the tube. I assume since it was a load KABOOM that it was
indeed the tire coming off instead of a pinch or some kind of failure
of the tube(Is that correct thinking?). I have not had this issue on
other rims but hese are a set of new to me(used with the used bike I
got) ALEX DV-15 rims. I partially inflate the tube. Insert it. mount
the other wire bead. Check valve is beyond tire bead and that tube is
not pinched on either sides. Partially inflate to 30 - 40 psi. Check
witness line. Partial Deflate. Inflate to desired level. Check. Sits
for 15 minutes or so off the bike before install. KABOOM.

I had this happen with a new folding tire(conti ultra 2000 28c) on
these rims and an old wire bead tire(ritchey speedmax 32c) that had
been on there previously. I just do not want to keep shredding tubes if
I am doing(or not doing) something I should. Nobody seems to have a
guide giving advice based upon failure mode. Searchs did not result in
anything about tires slipping off rims.