The reason for this was the attack of the Syrians on the detachments of Kurds, which allegedly took place to be. This was indirectly confirmed by the Russian Ministry of Defense, stating that the units of the Syrian army and their allies did not coordinate their actions with the Russian command.
A day later, information appeared about the allegedly dead soldiers of the private military company "Wagner" during a raid aviation USA. Such data were reported by several sources at once. It is reported that from 15 to 25 people died, about 50 were injured.
It turns out that the Americans are not afraid to attack the citizens of Russia. Formally, we cannot answer this, since all of them are not military personnel of the Russian army. But everyone understands that this can not continue.