Microstrip lines have significant energy outside the area outside the line and thus can easily interact with nearby objects. This is more pronounced when there are bends and twists in the lines. Unfortunately the drive to make antennas smaller often forces feed lines closer than one would like. Additional layers in a PCB to provide more isolation, stripline vs. microstrip, is often not a good idea for cost and increased complexity. Feedlines can also radiate and induce undesirable disturbances in the array radiation pattern. Arrays with microstrip feeds are notorious for this, particularly if they have been compressed by overall dimension requirements.