Apparently, there are pharmacological methods you can use to induce pon farr out of cycle, because some Vulcan couples are just *freaky like that*, only there's a small chance that the male will die of drug-induced plak tow instead of getting freaky if the dosage isn't exact.

I find it highly improbable that Spock would acquire these accidently. However, an enemy attempting to administer them without his knowledge... (especially, perhaps, an enemy who is unaware that he is bonded...)