I don't think one has to look beyond classic geopolitical dynamics to explain what's happening. Let's review ...
- Syria has long been a virtual Soviet / Russian client state that's now embroiled in civil war.
- Both Russia (historical / geopolitical grounds) and Iran (religious / geopolitical grounds) have vested interests in the Assad regime's survival.
- The Syrian opposition (represented militarily by the SDF alliance(s)) needs to be vanquished to ensure this survival.
- Kurds represent the largest single contributor / participant within the SDF forces.
- The century-old Kurdish question is a significant thorn in the side for Turkey, Syria, and Iraq.
- The spread of largely Kurdish military forces and territorial control beyond the Kurdish enclave in extreme northeastern Syria has arguably created a southern flank to whatever conflict one attributes to Turkey versus Kurds.
- Russia wants to foster more cozy relations with Turkey to reduce strategic risk (Bosporus), groom a Turkish regime that's now more to their liking, and wean Turkey's baseline allegiances away from the West and NATO.
- Russia also wants to reduce, repel, or eliminate the American presence / influence in Syria so as to score a political and strategic "win" validating their self-image as a major player in world affairs.
- The American president's abrupt call to withdraw from Syria immediately undermines the SDF and dims (more probably dooms) the Syrian opposition's prospects.
- The pro-Assad parties, in conjunction with the Turkish anti-Kurd campaign into Syria, can turn the tide against the opposition once and for all. In addition, Turkey may also reap gains against the Kurdish entities they want to eliminate along their borders and control within their own territory.
- It's a win-win for the parties who prefer the way Syria's been for decades, and a lose-lose for the parties who'd hoped for a new version of Syria.