I've seen a few kids (maybe one a year), who worry that parents/caregivers won't be there when the bus arrives, also sometime they worry that they won't get off at the right spot.
Getting to school is easy. Get on and get off when the teacher clears the bus. Getting home however is often more challenging. So have often worried that they will fall asleep and be on the bus forever.
In cases of bus fears I often ask an older kid at the same stop if they are willing to be the little kindergarten bus helper. To help that little one with the transition and learn the process. Also a quick chat with the classroom teacher and bus driver help.

Ask him if something is happening on the bus and raise hell if so.