I think there comes a point after many years of experience and observation when our pattern recognition kicks in and is undeniable. Our minds can only dismiss so much. At some point our survival instincts override the indoctrination.
For instance, there are only so many bad elders that can be written off as exceptions. There are only so many doctrinal changes that can be explained away as "new light". There is only so many times a person can handle false prophecy and have it excused as "maintaining urgency". There are only so many scandals that can be brushed under the rug. There are only so many meetings and conventions you can attend out of duty knowing the only way you can get through them is to daydream or get an assignment that takes you away from your seat.
Even with intentional suppression, you can only ignore these things so long. At some point your subconscious, driven by survival instincts and pure logic, spits out a warning message you can't ignore.
The proverbial damn breaks.
You suspect something is amiss and no "wait on Jehovah" or "maybe Jehovah is allowing this for a reason" is going to work this time. The logical part of your mind is winning market share and you realize that Jehovah isn't scared of critics or at least shouldn't be. Even Jesus apparently spent the day with the greatest apostate of all time, Satan himself. He spoke to him. He wasn't afraid. Surely our "truth" can stand up to some online research, for crying out loud.
After about a month of omg's and wtf's, you know in your heart of hearts, it's over. Decades of belief gone in a few weeks of listening to the counter-arguments.
Again, with the logical part of your mind gaining strength, you know that decades of "spiritual instruction" shouldn't fall apart in a matter of days.
Most of us do at least some verification of what we have learned to make sure we aren't being misled. Once we are satisfied that it is all true, we then know exactly why the Watchtower is so against looking at apostate materials. It is in their best interests, not ours.
IOW.....we were scammed.