Éric Prodon's ranking drop in 2005 and 2006 can be attributed to several factors commonly affecting professional tennis players. While specific details about Prodon's personal circumstances during those years are not extensively documented, general reasons for a decline in ranking could include:

1. Injuries: Physical injuries are a common issue for athletes, affecting their ability to compete and perform at their best. Recovery periods can lead to a significant drop in rankings due to inactivity.

2. Performance Slumps: Athletes may experience periods where their performance is below their usual standard due to various factors such as loss of form, confidence issues, or changes in coaching strategies.

3. Competition Level: The level of competition in professional tennis is high, and maintaining a top ranking requires consistent performance. Any dip in results can quickly affect rankings.

4. Personal Issues: Off-court issues, such as personal or family matters, can impact a player's focus and performance.

5. Changes in the Tour: Changes in the ATP tour schedule or structure might affect a player's ability to participate in certain tournaments, impacting their ranking points.

Without specific information on Prodon's situation, these are general possibilities that could explain a drop in a player's ranking during that period.