Here is the rule at least based on umpire bible... It didn't look to me that an attempt was made to avoid collision. He isn't forced to slide, but could have attempted to go around or slide to avoid direct contact. Any attempt to avoid the collision and he's safe due to obstruction. 1. The runner must make an actual attempt to reach the base (plate). Penalty: If the runner attempts to dislodge the ball or initiates an avoidable collision, the runner shall be declared out, even if the fielder loses possession of the ball. The ball is dead and all other base runners shall return to the last base touched at the time of the collision. 2. The runner may not attempt to dislodge the ball from the fielder. Contact above the waist shall be judged by the umpire as an attempt by the runner to dislodge the ball. Penalty: If the contact is flagrant or malicious before the runner touches the plate, the runner shall be declared out and also ejected from the contest. The ball is immediately dead and all other base runners shall return to the last base touched at the time of the collision 3. The runner must attempt to avoid a collision if he can reach the base without colliding. Penalty: If the contact is flagrant or malicious after the runner touches the base (plate), the runner is safe, but is ejected from the contest. The ball is immediately dead and all other base runners shall return to the last base touched at the time of the interference. If this occurs at any base other than home, the offending team may replace the runner. If the contact occurs after a preceding runner touches home plate, the preceding runner is safe. The ball is immediately dead and all other base runners shall return to the last base touched at the time of the contact. 4. If the runner's path to the base is blocked and (1), (2) and (3) are fulfilled, it is considered unavoidable contact.