Due to risible metagaming.
Again I disagree. The PC just cast the spell on the wrong goblin. "When you get the sword, throw it backwards." Job done.
Yeah. That's about the long and the short of it right there.The way I see it is you basically gave the goblin a free save against an effect he already failed to resist. That really is snatching a win from the players (even if relatively minor), so I can see why they'd be upset.
Due to risible metagaming.
To be fair, it hadn't gotten to a chase scene. It was a combat (three way combat at that) where the goblins were trying to get away from both the party and the undead guardians. And everyone was chasing the sword. In game it actually looked okay as they were scrambling over terrain, sometimes one goblin would take an attack, and not double move, that sort of thing.
Like I said, that part of things wasn't something I was having an issue with. It was just an odd interaction between the Suggestion Spell, Initiative and plausibility.
Although, thinking about it, I kinda screwed over the caster, who succeeded on casting the spell, so, I probably shouldn't have taken that success away. Funny thing is, the Suggested Goblin wound up dying before he even actually got an action. I was going to have him take the sword back from the other goblin, by force if necessary, and throw it back.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.