It's probably because it's in slow motion, it looks more natural at 2x speed

you're right, it does feel more natural at 2X,  but still that smoke plume at the point of origin looks odd as it spreads on top of that , due to the way it's lit up , kinda still feels like a bad CGI even when it's real , maybe it's the lighting that makes it seem a bit manufactured.

@ZeouLs : Lol damn!  that brutal accuracy . #Krakatau

btw can someone explain me the lightning? and why didnt squidward produce lightnings too

it's just the same thing with the clouds we see in regular , charged particles give of static in a big way after massive amounts friction build up by particles in the cloud of we see as lightning.  volcanic eruption being very quick forced expulsion of gases & dust & other debris alltogether , it creates the same effect much quicker than the regular thunder clouds which takes it's time charging up with slow atmospheric temp changes & winds & pressure diff .but with that said conditions have to be right, these kind of lightning are rare to capture & see .  we don't see it in majority of the volcanic eruptions