Hydrostatic pressure on the outside of the walls(can cause walls to crack/bow/buckle) caused from tons of dirt/clay which expands `n contracts, and--or roots from trees,and-or if there is a porch/addition is on the outside of this section of wall and wall can eventually crack from the footings for porch etc leaning up agst the outside of wall.....settling, and-or from when the wall was backfilled,it may have cracked on the "outside" when wall was backfilled (peastone is best backfill agst the outside of any wall) and on block walls..just because you didnt see/notice a crack on the inside a few years ago doesnt mean there wasnt a crack on the outside(yes, block walls can and do crack on the outside and do not show on the inside/not visible)...and its possible it was cracked when backfilled/or settling in that area and over the last 1-2 years the crack has...widened a lil more on outside(pressure) causing the crack to eventually...show on the inside