You may see your score drop when you have not completed any trips recently.
Although drivers have not made trips, scores still have the ability to change over time. This is not due to the idle driver's statistics changing, however, more so due to others within the team, continuing to drive causing a change to the underlying statistics.
In the case where a driver’s score has dropped, this would indicate that the population of drivers within the team are driving better overall. In the case where a driver’s score has increased, this would indicate that the population of drivers within the team are driving worse overall.