The Juncos Hollinger Racing driver spun at Turn 1 and stalled before restarting his car and driving into the path of Ed Carpenter Racing’s Christian Rasmussen.
Grosjean gave Rasmussen nowhere to go and the Danish driver piled into the Frenchman.
Rasmussen’s front wing broke and Grosjean copped a puncture.
The #20 limped back to the pit lane for a front wing change while the #77 trundled around and went off into the grass two turns later.
Rasmussen finished two laps down in 26th while Grosjean was three laps down in 27th.
The ex-Formula 1 driver received a drive-through penalty for unsafe conduct.
“That was lazy, that was foolish, to not look in your mirror and know what was coming and turn,”
said James Hinchcliffe on commentary.
The former IndyCar driver winced as Grosjean rejoined the race track in front of Dale Coyne Racing driver Toby Sowery.
“He just did it again. That was sloppy.”
Rasmussen was left to lick his wounds having started 16th.
“A day that could have been,” he said.
“I think we could have had a Top 12 today. Unfortunately, sometimes people don’t look where they are going and I got hit.
“We had to come in and change the front wing and that put us a lap or two down. It was kind of over from there.
“Looking beyond that, it was a pretty good weekend. Super close qualifying, we were right there with everybody, then good pace in the race and making moves.”
IndyCar continues on September 1-2 with a doubleheader at the Milwaukee Mile.