Could that "grass" be when the actual NMEA data is being sent out of the QLG1? It seems to last about 0.5 to 0.6 seconds and might be present while the 1PPS signal is high, but we just can't see it. Could be cross-talk in the cable or on the board itself. Since the amplitude is so low I wouldn't expect it to be picked up by whatever is monitoring the 1PPS signal but anything's possible.