Could it be: seat clamp not tight enough?

Clean the post, then torque the seat clamp to spec (usually 5–6 Nm). Do not exceed the marked limit.

Could it be: saddle rail clamp loose?

Torque the saddle clamp bolt(s) under the saddle.

A dropper that sinks on its own often has too much cable tension.

Troubleshooting › Comfort & fit

A seatpost or saddle that moves is a fit problem and a safety problem.

Seat clamp not tight enough · Most likely
Clean the post, then torque the seat clamp to spec (usually 5–6 Nm). Do not exceed the marked limit.
Fix: Pull, clean and grease seatpost
Saddle rail clamp loose · Most likely
Torque the saddle clamp bolt(s) under the saddle.
Fix: Set saddle angle and setback
Dropper post cable too tight · Sometimes
A dropper that sinks on its own often has too much cable tension.
Fix: Adjust dropper cable · Dropper post clean and lube