Just out of curiosity, why would a wall stand up for 18 months then fail? What are the builders doing or not doing?
Poor foundations for the soil conditions. I can't recall exactly which way round it is, but I think clay soils are prone to heave (lifting up) and contraction depending on the amount of water (or nearby trees, which suck up a lot of water), so you need about a metre depth of concrete for the footings. If the soil swells and lifts an insufficient footing, your brickwork will be lifted unevenly and crack.
It's something you hear reported on new-build estates, so it's not strictly a cowboy builder issue, just skimping on time and money. But I've seen it on extensions where the original house is 'settled', but the new extension isn't, so it moves and splits apart from the main building.
Edit: 90cm depth is the minimum for footings in England; I think in some parts, especially the south, you need to go down to 2m to get acceptable footings.