It's not just "passengers on Aeroflot". It's an old airtravel tradition that lots of people find to be quite wholesome.

Do you say a polite "thank you" to a cashier? Well that's a similar thing - thanking aircrew for their labour.

https://enroute.aircanada.com/en/aviation/clapping-when-plan...

https://travel.stackexchange.com/questions/68259/when-and-wh...

https://www.stuff.co.nz/travel/kiwi-traveller/131118567/flig...

https://www.skytough.com/post/why-do-people-clap-when-the-pl...

https://executiveflyers.com/why-do-people-clap-when-the-plan...

Some people find it irritating though, but who cares about these boring prunes.