From what I get, the ghosts can phase into the real world when in the presence of a person capable of seeing them. That's why she was able to trap the little guy in the sinkhole, because in the other world, there is only solid ground and no hole, and as she ran away and got far away, the guy couldn't be phasing into the real world once again.
Same here, there were 2 people on the bus who saw the guy, so he phased into the real world around these parts.