In the absence of any other clues, you probably fitted the new cap reverse polarity (impossible to prove one way or the other now the can has blown off), but it takes a major insult for them to blow like that rather than vent.
From what you say, it sounds as if the old cap was just failing in the usual way, drying out -> causing high esr -> causing high internal pressure. You say that the waveform was spikey with both caps. That implies that the rail was still outputting DC at that point (you would have noticed AC), so the diode probably failed after the measurement due to the reversed capacitor being a virtual short.
Sorry, that's the best diagnosis I can make from the available information. It sounds as if your eyesight had a lucky escape.