Official rulings aren't in yet, so this is all wild speculation, but the dominant hypothesis appears to be that these new planes are equipped with MCAS ("Maneuvering Characteristics Augmentation System") software that is supposed to account for inherent aerodynamic instability in the design by making it much harder for the pilot to accidentally crash the plane.

Without going into too much detail: there are many tradeoffs in airframe design, but one of them is fuel efficiency vs. inherent stability. Some airframe designs eke out a bit more miles-per-gallon at the cost of having ranges of motion where if the pilot gets the plane into that state, the feedback loops on motion become positive and the plane is likely to tip over, stall out, spin, etc. Boeing's Max-8 design makes this tradeoff, and the MCAS system is supposed to help compensate for this design by forcing the pilot to nose the plane down (to improve airflow over the wings and generate more lift) if the plane is pitched too high.

Problem might be that MCAS can malfunction and think the plane is pitching up when it's not, and if the pilot can't figure out how to disable it, they get very confusing signals--- they're pulling back on the stick to get the plane to climb, but MCAS is pushing the stick forward to avoid an expected stall that a malfunctioning sensor is telling it will happen. End result: pilot and the computer fight each other while the very important business of "keeping the plane in the air" isn't done and the plane crashes.