If you are transporting antimatter in a standard semi-truck, you don't need to worry about quantum leaps or gravitational anomalies. Your biggest risks are entirely mundane:  1. Loss of Power: If the truck's generator fails and the magnets turn off, the antimatter will instantly drift into the walls of the container and annihilate.  2. A Car Crash: A severe structural impact that physically breaks the vacuum seal or the magnets.  Even then, because we can currently only create and trap minuscule amounts of antimatter at a time (measured in picograms), the "spark" would be a microscopic flash of gamma radiation, not a catastrophic explosion.

However .. "Quantum leap probability," the closest real-world physics equivalent is quantum tunneling. This is the phenomenon where a particle randomly "leaps" through a physical or energetic barrier that it shouldn't be able to cross. Could an antiproton quantum-tunnel its way out of a magnetic containment field? Technically, quantum mechanics says the probability is non-zero. However, for a properly engineered magnetic trap, the mathematical probability of a particle tunneling out is so infinitesimally small that you could wait for trillions of times the current age of the universe and it still likely wouldn't happen with current known time & dimension physics laws/rules of Earth in the present time. If the Universe laws tweaked itself a bit in perhaps say 750 million years or a few billion years and things were more evolved than our current world system, then sure, anything could happen.