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.
Comments 2
Yes, that's what I expected. It happens because when you light the TNT with a flint-and-steel or a flame bow, the TNT essentially gets classified as "your" TNT – as a result, any mobs killed by the TNT will drop experience, add to your personal stats, etc. When TNT is activated by redstone or dropped by a dispenser, it can't attribute itself to a particular person, so mobs killed by it won't drop experience (and Guardians won't hurt anyone with thorns).
As I said in the description, if the Guardians dealt damage when a player damaged them through any means (sword, bow, tnt, etc) then it would make some sense because it would be consistent. It doesn't make sense, though, to be able to block Thorns damage with a bow, but not with TNT.
I have tried to reproduce 3 times:
1. I spawned some guardians and placed tnt. I activated the tnt with redstone.=> I didn't get damage.
2. I spawned some guardians and placed tnt. I activate the tnt with flint and steel.=> I got damage.
3. I spawned some guardians and placed tnt. I activate the tnt with a flame arrow from a enchanted bow. => I got damage
The activator will get that damage