A common cause is the vehicle reaching maximum adaptive pressure, due to slight slippage. This may also set a code P1870 when it occurs, thought the check engine light may not come on. If this is the case, cycling the ignition will normally clear the problem temporarily.
When cold the fluid is thicker and line pressure may be higher. This could prevent the situation. When hot and after driving a long distance the computer continues to boost pressure, if it feels slip. When you stop this will cause the clunk on shifting.
Diagnosis is simple with a pressure gauge, scan tool and a bit of driving. Repair can be as simple as an electronic pressure control (EPC) solenoid or as involved as a rebuild. Continued driving will normally cause damage and make the later necessary.