A go-around at very low altitude (a.k.a. wave-off or aborted landing) usually results in momentary touchdown; nothing strange about that. And engines need time to spool up; nothing strange about that either.
I guess you didn't actually witness a hard bounce, followed by a go-around, but rather a bounce during a go-around - quite a different sequence of events (and certainly not as dramatic). Remember also that the sound of the engines spooling up will only reach your ears a couple of seconds after the actual event.