A leaning retaining wall is one of the most common problems we're called to look at around Waterbury, and the cause is almost always the same: water. Soil soaked by rain or snowmelt gets much heavier, and if the water has no way out, it pushes on the back of the wall until something gives.
A wall that lasts starts below ground. The first course of block sits on a compacted gravel footing, set perfectly level and partly buried so the wall can't slide forward at the bottom. Skipping this step is the quickest way to get a wall that looks fine for a year and then starts to move.
Behind the wall, we backfill with clean drainage stone instead of the dirt that came out of the hole, and run a perforated pipe along the base to carry water away. That drainage zone takes the pressure off the wall. Taller walls also need the blocks stepped back slightly and, depending on height, reinforcement grid tied into the soil behind it.
If your wall is already leaning, bulging, or has cracked caps, it's usually better to rebuild that section properly than to patch the face. A rebuild lets us fix the footing and add drainage, which is the only thing that actually stops the movement.