One can only presume that Obi-Wan really did make some sort of failed roll here. Wasn't he lecturing Anakin a while back to be more careful with his laser sword, because, "This weapon is your life!"*

What sort of multiple failures would Obi-Wan really need to go through to drop his sword and not simply Force yank it back into his hand immediately? First he'd need to not have holstered it safely, despite being on a rapidly moving and unstable animal that requires both hands to hold on to the reins. Then he'd have to be so shocked by the landing of his lizard mount that he actually lets go of the weapon that represents his entire disciplined life as a Jedi. Then he'd have to have his superhuman Jedi reflexes fail to grab the sword back before it moves out of his grasp. Then he'd have to fail to think to use the Force to simply keep it from falling and return immediately to his hand.

Okay, make that multiple failed rolls. It's amazing he didn't also fall off the lizard.

* We mean in the actual movies, not the comic.

Transcript

Yoda: "Grievous falls from Favourite Position! Can plucky Jedi prevail?"
GM: He climbs into a vehicle and—
Yoda: "Exclusive! Cyborg Madman attempts Hit-and-Run in Giant Metal Fun Ball! Clone Trooper witnesses reveal all."
GM: Well, erm, make a Dodge roll.
Obi-Wan: All good. I'll call my riding lizard, and—
Yoda: "Boga versus Bike in High Speed Sink-Hole Chase!"
Obi-Wan: Sally, you can't narrate this whole session. We're trying to play here too, you know.
GM: Your mount dives straight down the side of a cliff.
[SFX]: Whump! {Obi-Wan's varactyl lands hard}
Yoda: "Jedi Ponders Futility of Existence, drops Laser Sword in midst of Perilous Pursuit!"
Obi-Wan: What?
GM: She has a point. Make a Dex roll to prevent the shock of landing from jarring your sword loose.
Obi-Wan: <grumble> 4.
Yoda: "Swords from Heaven! Baffled Clone Trooper Retrieves Jedi Weapon!"
Obi-Wan: Okay, that's enough, Sally.
[SFX]: pow
[SFX]: clang
Yoda: "Stares of Death! Local Roleplayers Awed, Angered by Prococinous Talent!"