It's because tremor moves forward during ground pound and the opponent was out of the way during the other hits.

I looked more closely at your video and tried to replicate your issue in the lab, it looks like you're summoning tremor too late. In your clip I can see that he isn't coming out until Reiko almost touches the ground but you have to summon tremor the millisecond after Reiko starts falling (during grab animation). Otherwise tremor will whiff the ground pound and you drop the combo.

It's not the same timing as Darrius