The rider drew a wicked serrated blade but his horse collapsed and San'doro ripped him open with a violent backward tug on his left dagger.
The man fell off his horse as it fell to the ground. 
mnli hypothesis: The man fell off his horse as it fell to the ground.  premise: The rider drew a wicked serrated blade but his horse collapsed and San'doro ripped him open with a violent backward tug on his left dagger.