Wouldn't the most likely candidates cancel each other out?
In a surprise succession, either faction fight it out or, neither of the likely Stalin cronnies being tough enough to prevente a military take over, a deal is made and a relative outsider chosen as a compromise candidate.
Well "likely" candidates have to overcome the concrete institutional realities. Stalin didn't come to power because he was extremely well liked by every section of the Party. If that was all that would be needed then Bukharin would have become the leader of the party. He also didn't come together because he had the best theoretical knowledge, was a great orator, or was great at political agitation. That describes someone along the lines of Trotsky and he was ejected from the party. Stalin overcame all his political opponents because of his institutional power and his organisational clout. He was appointed head of the Uchraspred in 1920 and the Orgbureau in 1922 - both were organisational institutions that allowed the promotion and demotion of members, the selection of candidates for congresses, the transfer of party activists to different parts of the country (allies to Moscow and opponents to Ukraine for example), amongst other things. This allowed him to build an institutional network that was reliant on him and allowed him to outmanoeuvre his political opponents. A very boring bureaucratic form of politics that overcame the more politically bombastic vibrancy of others.
Molotov was Second Secretary at the time, was a full member of the Secretariat, the Orgbureau, and the Politburo, and the Central Committee, as well as being on the Presidium of Comintern. He was a close ally of Stalin and held, second only to Stalin, much of the institutional power that allowed Stalin to control the party. Whether Molotov can actually levy this institutional power is another thing as perhaps he wasn't as ambitious or ruthless as Stalin was but still that's a lot of weight in his corner.