Is there a technical term that refers to a knot untying under load because the tail was too short?
This is called rolling, inverting, or capsizing. What happens is that the knot turns inside out, and part of the tail is consumed. If this happens repeatedly, or the tail is short, you can use up the whole tail, and the knot fails.
For example, an offset figure-eight is not a safe bend for use in tying two climbing ropes together for a rappel, because it can roll if you don't dress it carefully. There have been several deaths caused by people using this knot for this purpose. An offset overhand does not roll, so it's the preferred knot for this purpose.