The danger that sovereign spreads would spiral away, undermining monetary transmission as well as a lot of other things, seemed real, especially as the funding firewall that was quickly agreed at political level would take several months to become a reality. ECB intervention in secondary debt markets looked like a sensible and urgent action needed to nip things in the bud.
A moment analogous to that seized by JP Morgan in 1907, if you will. For some, the SMP may have looked like something close to the dreaded monetary financing, but the Treaty is pretty clear on these matters, and buying plainly in the secondary market is not monetary financing.