TNT Minecart doesnt prime correctly

further experiments show, its only from the falling height shown in the video.

Intended. The cart just falls through the water an extra block (and therefore has fallen enough to explode).

okay... so how is it getting activated without a red stone signal on the activator rails?

TNT minecarts explode when they fall far, crash into something, or go around curves at high speed.

okay, well this is settled then.