Some bar carts are very heavy, 40 Kgs or so and require a fair bit of effort to get up hill or strength to prevent them from going down hill. Which is why we keep the deck angle to less than 10 degrees when the cabin crew may be out with their trollies. And loose carts do cause very unpleasant injuries. Reasons for them becoming loose include faulty or non-applied brakes, extreme deck angles, poor stowage and loss of control (small CA, big cart).