I think the main reason his supporting suddenly got popular is because of the riptide/shield changes mixed with Cinderhulk putting tank junglers into the spotlight. Previously, all of Nauts power was in his shield and his riptide was nothing more than a strong slow. Now his shield can be used more like a shield and his riptide is his damage now.
Naut Support, in my opinion as a fellow Naut player/pseudo-main, has always been good, with some variation depending on the popular ADC at the times, screw Lucian and his slow-breaking dash and minion wave advantage... But with Riptide being his new bread and butter his entire shtick isn't hook in, shield, riptide, and hope your ADC isn't playing Lee Sin and actually follows up on your 3 seconds of CC because now you're useless for 10 seconds. Now it's hook in, shield, and go to town on the ADC as their support peels you off as their ADC has to decide between hobbling away (and taking multiple riptide hits!) or counter engaging which, with a Naut sitting on top of you, probably isn't the easiest thing.
TL;DR, Shield/Riptide power shifts put him on a better power level with other tank supports, Cinderhulk put him in the spotlight.