If the brace isn't tapered at the end, ending abruptly, it retains its stiffness there. If there's stress on the top at that point, from pressure or an impact, it can cause the end of the brace to separate from the top. The stress can cause the top to sort of peel off the brace, sometimes taking some of the brace with it. If the brace is thinned down close to the top at its end, the end can flex with the top, helping to avoid separating. It can also contribute to a top cracking. It's less of an issue with thin braces which can flex more readily with the top, as most fans are that I've seen in my limited experience in the CG world.
Another issue arises in the active area of the top, say below the bridge, where a gradual change is stiffness is often the goal. An abrupt end of a brace creates an area of concentrated stiffness, which many think is detrimental to sound.