i experienced something new to me this weekend and i was hoping you can confirm my guess as to what caused it.

Typically i back the trailer into the water to a depth that requires just a slight push from the outboard in order to get the boat up the trailer bunks.  We're talking about a foot at most.  I do this rather than having the trailer further in the water and the boat floating above the bunks.  it helps in getting the boat properly aligned. of course the motor is trimmed up a bit when doing this.

But this weekend i launched from a very steep ramp and one i have never used before.  when i was preforming the above procedure and giving it a little throttle i noticed the motor start to "trim" down on it's own, rather quickly too.  i immediately let up and raised the motor again.  tried it again with the same results.  I gave up in fear of screwing something up and backed the trailer up a bit further and manully ratcheted the boat by hand while in knee deep water.  There were several gators around and this is the same canal (Dead Canal, Lake County) that a kid was killed a few years ago, so i wanted to stay out of the water as much as possible.

Was the shallow and steep ramp creating an undertow effect when giving the motor some gas and pulling the it down?

Sorry if this is a no brainer, it's just new to me.

Because you're trailer was on a steeper grade, the engine was required to invoke more horsepower than usual to get the boat up onto the trailer. The power tilt will only hold the motor out until the force of the engine pushes it back in.  It happens most of the time I put my boat onto the trailer.  Nothing to be concerned about.