Why does An-225 (Mriya) perform a low-pass on its landings?
As of the time of writing this answer, this only appears to have happened in one out of the last 6 flights of the AN-225 (the same flight into Warsaw you mentioned, https://www.flightradar24.com/data/aircraft/ur-82060#245b19c0) so I can't say that this is an official "rule", a standard of any kind or any procedure regarding the AN-225.

There are 2 other videos of that particular landing on YouTube:


                
            


                
            


It is clear in both videos that the first approach is done with the landing gears up, most likely on purpose "for the show", of course with coordination with ATC. 

If someone is fluent in Ukrainian they can confirm it quite easily since the communication between the pilots can be clearly heard in the video you provided.