Why the breaker trips

[kwired]

They did make QO with aluminum bus. I believe they still do in smallest sizes

Those 1970's QO panels with the 100 amp main that spanned all the way across both sides of the panel were typically panels with aluminum bus. If there is a problem with them it often is burned bus to breaker failures. One of their mistakes with that product line. The six and 8 space load centers with horizontal bus are commonly sold with aluminum bus, but is also a catalog number for ones with copper bus, probably tin plated so won't be obvious just looking at it to tell the difference.

 

[EngineeredElectric]

I found a Seimens last week in a QO, I did not leave it.View attachment 2582081

Siemens makes breakers that are approved for use in QO panels