Why do cosmonauts piss on a wheel of the bus transporting them to the launch pad?

Apparently it's a tradition for cosmonauts to urinate on the rear right tires of the bus transporting them to the launch pad. Russian cosmonaut Sergey Prokopyev reportedly did just that Wednesday before the Expedition 56/57 crew took off for the International Space Station. While male cosmonauts release the stream directly from the source, females carry a cup of urine that they pour onto the tire. Space.com explains why:

(They are) paying tribute to the first human in space — Yuri Gagarin. The cosmonaut, who launched April 12, 1961, from the same cosmodrome, had to "go" on the way to the rocket … and the rest is history.

Various other Gagarin tributes also come into play for launch crews — they also visit Gagarin's grave in the Kremlin Wall Necropolis in Moscow. And after arriving in Baikonur, they plant a tree in the same grove where Gagarin planted his; and they visit his office, which has been preserved since his death in 1968.

"Why Cosmonauts Pee on the Bus That Picks Them Up for Launches" (Space.com)