I want to know this too. I/K have their motives but S/Z? I want a quote from Sinitsina because I cannot imagine her (or even Zhiganshin) wanted a split.
Of course S/Z have motives. S/Z have been at their best third Russian team, and not even settled third, because until this season Riazanova/Tkachenko were third and S/Z would be fourth at best. By getting each of them one half of I/K, they managed to get better achieving partner than they had, and completely shake up Russian dance order. Suddenly Russian spot 2, 3 and 4 is open, and both new couples will be fighting with Riazanova/Tkachenko for second and third spot, with better partner than they had. It is one of I/K that has something to loose because one of them will suddenly drop from secure second spot (which was very close to first spot - they did beat Bobrova-Soloviev sometimes) to third spot.