This is my speculation: but we know they didn't follow their orders to topple Syrian government to the letter and decided to send part of their forces to Iraq instead (which was already in US control) and thus US decided to clean them up from that area.
Alternatively, they already fulfilled their objective by destabilizing and breaking up Syria (US took control of their oil fields and some border crossings) and they were then cleaned up from same areas (but not from areas which would fall to Syrian govt). Isis fought against US during those cleanups maybe they think US betrayed them).
Are you referring to that or something else?
Someone who has followed the war more closely should also offer their interpretations about this.
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