I don't really think Everton fans are unique.
Glancing at the Guardian's MBM report of the West Ham - Leeds game earlier, it mentioned how the atmosphere at the London stadium had greatly improved of late along with the results and performances. We even got a bit of a name check when saying that the atmosphere before Moyes had been up there with the 'Goodison groan'.
Imo, all fanbases are reactive to what they're seeing on the pitch at any given time and to what they've witnessed (or heard/read about) in the past. We've had almost 3 decades of relative mediocrity. Any fan will groan seeing the first misplaced pass or defensive mix up after 40 seconds if the previous 5, 10 or 15 matches have been full of misplaced passes and defensive mix ups. Whereas, if the team usually starts slowly but gradually gets into the game and always competes, there'll be roars of encouragement after the first mistake
If you watch the rs ( as a lot of you seem to
) losing at home to Watford, there's a good chance the fans will be defiantly singing because they're pretty sure that the next game they could beat Chelsea 3-0. Why, because they've had repeated experience of that kind of result. There've been more good times than bad times for 2 generations.
However, if they lose that next game to Chelsea, then draw with Southampton, then lose to City, if they're then losing at home to Wolves, the groans will start.
So now put Liverpool or Utd or City in a position where they haven't won anything, anything at all, for 26 years, often watching truly awful fare, and you'd hear the groans every week.
When negative expectations outweigh your positive hopes, it's rough.