1. Estrus. During the breeding season, especially when females are in estrus, they may rub their heads against the heads of other turtles, which can help them find suitable partners.
2. Fight for territory. Turtles may compete for territory by rubbing their heads against other turtles' heads. This behavior is common when one turtle tries to occupy another turtle's shell or an area. At this time, there may be conflicts between turtles and they need to be raised separately.
3. Social interaction. Rubbing other turtles' heads with their heads can also be regarded as a kind of social interaction, which is common in establishing and maintaining friendship between turtles.
If this behavior persists and another turtle shows discomfort, it is recommended to consult a professional veterinarian or animal behaviorist.