@whataboutbob1"Its pure speculation, but adding the new components could have stressed out the older UCC’s. I don’t know for sure, but it’s quite possible. The original power transformer that was in it had a direct short and could have caused the UCC’s to stress."
Does that sounds like a reasonable explanation?
No- its the other way around.The failing filter cap drew enough current that the old power transformer eventually failed (a short is a typical failure in this case). Since they didn't replace the filter cap, with the new transformer (which apparently is a bit more robust) it was able to sustain its current- and the defective filter cap overheated and leaked.
Just so you know, the mechanism of filter cap failure is that when they get old enough, they can draw a little bit of current (normally the draw none of their own at all). This current causes the cap to warm up. As it does so it will draw more current. The insidious thing about this is it might take quite a while before the cap is drawing enough current to cause damage- but in the meantime the transformer is running hotter and hotter until eventually the insulation inside melts and it shorts.
he said the only way for them to explode is if they were wired with reverse polarity.This statement as you can now see is false. It is true that if you wire them backwards they'll draw a lot of current and explode. Usually they do that pretty fast. But if the part has failed it can do this as well- the difference being that it usually takes longer.
IMO/IME they should have replaced the filter cap at the time they replaced the power transformer. Transformers should last decades- the only die if overstressed, and filter caps can do that to them. Any competent tech that saw a failure like that would have been asking why it happened and looking for the cause.