The balance changes for League of Legends ARAMs are here ready for patch 11.13. The for-fun game mode set on the Howling Abyss gets its own balance changes due to how Champions will perform differently on the unique mode.
The new Dr Mundo is likely tearing up the Howling Abyss, especially with all his defences and healing, so increasing his damage taken will help negate his strengths. Zyra, too, is very safe due to being able to use her plants from long range; so a nerf to damage dealt and an increase to damage taken will help bring her down to a more healthy state.
Melee Champions, such as Udyr and Hecarim, regularly get buffs to help them as they need to close the distance on the narrow map. Reducing the damage taken helps them to do that, and increasing the damage dealt rewards them when they get there.
The crown jewel is Warmog's Armor. The item regularly has been seen as unhealthy for the game, allowing tanks to quickly heal to full health in between trades; very frustrating for opponents who can't outright burst them and somewhat defeating the purpose of the game mode where players can't return to base to heal.
The latest changes to Warmogs only made it worse, as the lower health requirement made it more accessible. However, with higher cooldowns it makes the healing more manageable. By being able to poke the tank, players can more effectively stop them from healing up again.