What causes a gap after getting a crown?

Teeth shift: Teeth shift slightly due to how the opposing teeth hit them causing orthodontic like forces that can lead to a crowned tooth shifting and a slight gap openning.

Short contacts: When the crown if fabricated, occasionally the space between the crown and the adjacent tooth is not filled in completely. If this is causing food to get impacted between the teeth, the crown may need to be remade. Usually the dentist and the lab he/she works with will do this for you without additional charge unless there is a complicating factor.