Many years in the auto service industry and I've seen this issue in several Import and Domestic models both passenger car and light trucks. Strangely enough, none have ever brought them in with that complaint wanting to have that "issue" addressed/fixed.
The most recent was in a '02 Mitsubishi Montero Sport ( drum rear ) the first few brake applications were, as you said, scary. The slightest braking would result in a severe locking up of the rear brakes, and I mean just touching the brake pedal. The cars are typically left overnight for other repairs and the locking issue would diminish by the time we pulled them into the service bay. The thought would be "I'm sure I can sell a rear brake job on this one" to our surprise that on inspection, no excessive/visible/easy to sell/ wear is ever noticed.
I say all this to let you know that it's not too rare of an issue. Most people get used to it and don't want to spend the money to fix it. That being said, my guess would be drum and shoe diameter difference exceeding specs or maybe no chamfer on the friction materials edge if you don't find anything obvious. I would be very surprised if it were a hydraulic issue.
Sorry that I can't offer a solution. I will be watching for others smarter than me to chime in with the "Fix".