Guardians have a Thorns effect, so when damaged they deal a bit of damage back to their attacker. This happens when hit with a sword, but not a bow (which is logical).

However, it also occurs with TNT, which doesn't make sense. Switch to survival mode, then find/spawn a guardian. Place TNT near it, and light it with a flint and steel. Then retreat out of range of the blast.

The player who lit the TNT will take Thorns damage, even though he might be dozens of blocks away.

If the thorns effect applied to every kind of damage, it would be consistent and a bit more reasonable; at the moment, though, it applies to TNT and not to a bow, which doesn't make any sense.