I once had an Orgo TA refer to a diazo compound as "diazo-boom-boom" (the technical term). I have always been curious as to the reason behind the instability and reactivity.
According to Wikipedia
Some of the most stable diazo compounds are α-diazoketones and α-diazoesters since the negative charge is delocalized into the carbonyl. In contrast, most alkyldiazo compounds are explosive
What is it about the alkyldiazos that makes them so much more unstable? There doesn't appear to be any bond strain or other factors. Naively, I can say that having a resonance structure should make it marginally more stable.