Bursting tyres with heavy loading

I've been riding to uni on a clunker of an old mountain bike, using
that instead of my Trek roadie because I could mount a rack to it and
carry panniers.

I don't think I've really got all that heavy a load in there. A laptop
computer, change of clothes and my lunch. On some days, a lab coat too
and possibly a thin plastic rain jacket. That's it!

The bike is a somewhat heavy one and I weigh about 105kg.

Today my brand new rear tyre and/or tube burst. There was a loud
"pshht" and then my tyre had no air left in it. It had been inflated
to 100psi as per the sidewall markings and the tyres are supposedly
"thornproof", for what limited good that is.

This tyre is a 1.25" x 26" thin (ish) mountain bike tyre.

A few weeks ago I managed to burst a wider (1.75" I guess) rear tyre,
but I blamed that on it being a very old one which had been squashed
under its own weight uninflated in a shed for a couple of years at
least and therefore dodgy. That one had been inflated to 60psi.

First of all, is it more likely to be the tyre or the tube which is
responsible for a popping tyre? The tube I bought off eBay is a Kenda
which I understand is a fairly well known brand. The tyres were these
Innova ones: https://www.torpedo7.com.au/products/INTYMNNPPXX125

Should I blame the tyres, and possibly ask for a refund from Torpedo7,
or blame the tubes and maybe try for a refund from the eBay vendor, or
is a bursting rear tyre normal enough when one tries to load them up
with a 100kg+ rider, a 20kg bike and 15kg of luggage?

In the mean time, I have a 1.75" tyre and tube in good condition. For
the loads I'm carrying, would I be less likely to see another burst if
I just patch my 1.25" tube and put the 1.25" tyres back, or should I
switch to the wider tyre for the back?

100psi is the "Max PSI" according to Torpedo7. Should I inflate it
less than that? How much less?

Travis