There are a few common reasons, your parcel is delayed. If you are already waiting, the best idea is to make sense of the information from the tracking.
We made a survey to write a glossary of a tracking status values, because there are so many for so many languages. Survey link
The main reason is congestion: Only so many parcels can be on a plane or ship. Usually budget shipping services are so cheap because they don't have priority spots or rely on latching on to unconventional transportation methods, that's why it varies more with them.
Another big reason is customs.