It's not water pressure which has caused that damage. It is either the weight pressing behind the wall which has 'pushed it over' or it's the lack of stability under the foundation concrete, i.e. the footings have tipped a little and obviously the wall follows. I'd wager it's the former.
Often walls like this are built with shallow footings and then loose backfilled behind, the backfill gets saturated and over the years it settles downwards and unfortunately for you, outwards. This pushes the wall over.
It probably should have expansion joints in it too, but even they wouldn't have stopped this damage. Expansion joints are to cope with a little seasonal expansion and contraction not physical damage.
Solution is to take down the parts of the walls which are leaning over and see what the footings are like, if you're happy that the footings haven't moved, then rebuild the walls with square ends and screw in slip ties. Fill the joint between new and old with a flexible caulk.
Something like this In this case the fire requirement isn't needed.
Carefully back fill the wall again. This is not guaranteed not to move again in the future however, that's just the nature of these "too small" retaining walls really. It takes a lot of strength to stop cubic metres of wet soil and a block wall just doesn't have it...