Without having seen your piece, of course, I can only speculate, but I wonder if what you were doing was the opposite of predictability: You signaled you were going straight, or right, when your goal was to go left.
I disagree that every book has to be a safe, predictable "the same but different," as Mark's agent said. There's certainly a market for that, but I wouldn't use it as a guiding principle for every story. What your readers may have been asking for was a flag that something was going on.
Let me use a TV show as an example, because I can't think of a written one at the moment. The BBC's wildly successful Sherlock aired its fourth season at the beginning of 2017, and it was almost universally panned. People actively hated it. The cinematography was gorgeous, the acting was wonderful, but the writing was... like we were watching the wrong show.
The previous three seasons plus a Christmas special taught the audience to expect a mystery or two per episode, clever banter, Sherlock as brilliant and pretending to be cold but actually very sensitive, John as brave and telling himself he's normal while chasing after excitement, Mycroft as a cold-blooded government spymaster, Mrs. Hudson as the affectionate and exasperated friend, Sherlock and John as the closest of friends (and probably in love with each other but unable to admit it), and a realism-based narrative.
But the fourth season went absolutely off the rails from all of this. Sherlock ignoring John, John barely speaking to Sherlock, Mary constantly in the middle of the two of them, Sherlock claiming to openly prefer Mary to John, John assaulting Sherlock and beating him bloody, Mrs. Hudson drag-racing an Aston Martin, Mycroft vomiting in distress after seeing violence, a mysterious mind-controlling secret sister in a secret government installation who is somehow in league with Moriarty who's been dead for years, an explosion which blows two men out a second-story window without a scratch and destroys the flat but not the rug, a character leaping in front of a bullet after it's fired and having a Hamlet-length death scene when it's established that a chest shot puts you out in three seconds... I could go on.
My point is that the fourth season of Sherlock was so inconsistent with everything else we'd seen in the previous three years that a large chunk of the online fandom has spent the last eight months trying to figure out What Really Happened, Because That Wasn't Real. Is it someone's fantasy? A mind palace vision? John or Sherlock is dying and hallucinating? Did reality separate at the end of Season Two?
At the end of The Reichenbach Fall, John leaves Sherlock's "grave" and the camera pulls back to show Sherlock watching John leave — so he's clearly not dead. That was the flag which the showrunners gave us so we knew something was going to happen. We didn't know what, but we knew it was something.
We didn't get an explicit flag anywhere in S4 to tell us "You are not supposed to accept these events at face value. Something Else is going on." Maybe Mofftiss didn't intend to tell us what's going on until S5, but they didn't warn us that we'd have to wait for S5. So we're angry and confused because we don't know where all this is going. Is it just bad writing? Is it layers of subtext? Is it meant to be symbolic? We don't know.
Perhaps what happened with your story is that you knew where you wanted it to end up, but you didn't signal to your audience that Things Are Not What They Seem. Some little bit of foreshadowing, signaling, flagging, or other direction was missing. Your readers need a hint about your endgame. The hint can be as vague as "Even though this is set up like a sitcom, it's not actually a comedy." You don't have to spell out on page 3 that it's going to be horror/drama/mystery/sci-fi/alternate history/southern gothic. You just have to let people know that their expectations are being deliberately subverted.