"too much" TNT in water deals environmental damage

https://www.youtube.com/watch?v=HYW7LVFFC3A&feature=youtu.be

In the setup pictured, in the 1.8 snapshots 4 or more pieces of tnt in 1 water block will deal environmental damage to it's surroundings.

This does not occur in 1.7 or earlier, thus cannot be the same bug as https://bugs.mojang.com/browse/MC-8565
This bug was introduced in the 1.8 snapshots, MC-8565 affects older versions such as 1.5 and 1.7. this bug does not occur in those versions. Please do not mark this as a duplicate of that bug.

This also happens without dispensers. Dispensers are convenient for showing the bug, but the bug does occur without using dispensers