Mystery of the Healing Tower

I was playing a game last night as Lone Druid. It was most of the way through the game, and I was focusing on ripping down the remaining towers, as I was a bit underleveled to face the other team face to face. There was only two towers outside the enemy's (Radiant's) base, and I decided to go for the center tower. As normal, I walk up to it, pop both buffs, and start pounding it. After a few moments, I stop and look at the health, and I notice it hasn't gone down very much. I keep watching, and I was shocked to watch its HP going UP rather than down.

I kept attacking it for a few seconds to confirm, and indeed, its HP was rising. I gave up, went off and attacked a different tower. We still won, but I cannot figure out why the tower's HP would be rising. There was no Treant Protector on their team, which is the only ability I know of that can heal towers.

Replay Data to come later when I have time to fetch it. I was curious whether it was some form of glitch or merely my ignorance as to other ways in which towers can heal.

Backdoor protection? if there are no allied creeps nearby then it will have very high regen to the original health that is was at.

It seems so. I didn't realize that was how the system works. Thanks.

Just so you know : The first tier towers do not have backdoor protection, wont heal even if there are no creeps.

Don't be discouraged! The backdoor regen is a fixed rate, and you can easily beat it in the late game. It's meant to make it harder to "backdoor", not to make it impossible altogether.

Also, for any buildings in the enemy base, the backdoor regen is disabled if you have creeps in the enemy base. For example, if there are creeps coming in through bot lane, there's no backdoor protection for top lane towers/rax.