There can be a number of reasons - cutting, stitching, ironing (instead of pressing) and sometimes just the mean fabric gods! But, one way to stop any problems is to cut your patches 2 3/4 rather than 2 1/2. Make your 4 patch, press and then trim to 4 1/2. This is a bit wasteful but, when you weigh it against the frustration and ripping out, it is money well spent.