Because they listened to everyone except the Russian and Syrian governments all these months. Instead they listened to the voices in their heads. And they made up lies about what was happening in Syria, and about Vladimir Putin and Bashar al-Assad, and came to believe their own fabrications -- the inevitable outcome for liars.
So for those who say they're confused by the announcement -- no. They're not confused because even when confronted yesterday with the collapse of their lies and fantasies they immediately voiced unfounded speculations and made up lies which they call 'theories' about why Russia is withdrawing at this time from Syria.
They are not confused. They still prefer the world of words they created to the truth, and believe their tongues are powerful enough to bend reality to their will.