On September 20 I wrote about the likely reason for the willful U.S. bombing attack on a critical Syrian army position in Deir Ezzor:

Two recent attacks against the Syrian Arab Army in east-Syria point to a U.S. plan to eliminate all Syrian government presence east of Palmyra. This would enable the U.S. and its allies to create a "Sunni entity" in east-Syria and west-Iraq which would be a permanent thorn in side of Syria and its allies. 
...
The U.S. plan is to eventually take Raqqa by using Turkish or Kurdish proxies. It also plans to let the Iraqi army retake Mosul in Iraq. The only major city in Islamic State territory left between those two is Deir Ezzor. Should IS be able to take it away from the isolated Syrian army garrison it has at least a decent base to survive. (Conveniently there are also rich oil wells nearby.) No one, but the hampered Syrian state, would have an immediate interest to remove it from there.

There are new signs that this analysis was correct.