The team was worried Harry might have died from so high a fall.
In addition to the unusual circumstances that caused Harry to fall, he’d also fallen from high up, and his teammates thought he had died from it. They were likely more concerned in this case than in other cases of falling off a broom because of how serious it could have been.
“It was as though Harry’s memory was on fast forward. The lightning … the Grim … the Snitch … and the Dementors …
‘What happened?’ he said, sitting up so suddenly they all gasped.
‘You fell off,’ said Fred. ‘Must’ve been – what – fifty feet?’
‘We thought you’d died,’ said Alicia, who was shaking.”
- Harry Potter and the Prisoner of Azkaban, Chapter 9 (The Grim Defeat)
When Harry just broke his arm, they don’t all seem this worried - the severity of this particular fall and their belief he might have died likely made them more concerned than in a typical situation.
Also, they were likely also shocked at the Dementors being there.
In addition, it’s also likely that part of the reason that Harry’s teammates are so shocked when Harry falls off of his broom is the way it was caused. He didn’t fall off as a result of any of the typical Quidditch hazards like Bludgers - he fell off because Dementors were below him and he’d fainted.
“And then a horribly familiar wave of cold swept over him, inside him, just as he became aware of something moving on the pitch below …
Before he’d had time to think, Harry had taken his eyes off the Snitch and looked down.
At least a hundred Dementors, their hidden faces pointing up at him, were standing below.”
- Harry Potter and the Prisoner of Azkaban, Chapter 9 (The Grim Defeat)
Therefore, it seems likely that at least part of the reason they’re all so shocked, in addition to them thinking Harry might have died, is that Dementors got onto the Quidditch pitch.