This one is quite difficult to determine.
When you watch it in slow motion it is a bit deceptive as everything seems more premeditated in slow motion.
It appears though that both players behaved in a reckless manner:
The defender jumped into the offense player and therefore their actions could be considered reckless.
The offense player was not watching where they were going, and therefore their actions could be considered reckless.
The basics of it though is that both players reacted very quickly to where the disc was and both made a play at the same time. So it would seem best to treat these sorts of collisions as Offsetting Fouls:
[i]17.10.1. If fouls are called by offensive and defensive players on the same play, the disc returns to the thrower.
17.10.2. Non incidental contact that occurs as two or more opposing players move towards a single point simultaneously should be treated as offsetting fouls[/i]