This is really common. It might not be anything about the gunners that scared him, my guess is he was running so well you gradually made the marks longer or more difficult until he hit a wall and his training for force and handling was not up to the same level so you couldn't do much about it but help. And hiccups like this take about TWO SECONDS to start and A MILLION YEARS to overcome especially if he's allowed to repeat the behavior over and over.
So what Mr. Nelson said is spot on, baby him a bit on marks for the next month or so, basically, ensure that he will be successful. No corrections or drama if he doesn't go. If he balks, just walk him half the distance to the mark and repeat.
But please know --- proper and thorough FORCE FETCH through pile work will give you the tools you need to finally correct this behavior. At some point the dog must complete the retrieve out of an obligation not because he feels like it. At this point in time you don't have the tools to correct him and convince him he IS going to continue to retrieve without help. Administering a correction too early in his career can majorly backfire, but not correcting at the proper time when he has the tools available to understand the correction, is just as bad, and wasteful.
Brix was pulling some of this nonsense, same sort of thing. It had absolutely nothing to do with the gunner. It had everything to do with being lazy and tired and that morphed into a superstitious habit. Which until he had the tools I couldn't do much about it but keep the marks short and exciting. In the past few months we've worked on FTP up to about 100 yards, forcing en route and back-nick-back. Just two weeks ago Brix was sent on a 250 yard mark, got out about 20 yards, and popped. Finally I could force him with back-burn-back and guess what he turned tail and tore off after the mark. Hasn't done that since. It's not cured but I have the tool now.
Best of luck. Don't give up, but don't PRACTICE bad behavior.
Thank you so much for this!!!