Hi, you say this is a mature lavender plant...does that mean it was actively growing (not dormant) at the time of transplant? Was the plant potted or was it dug up and relocated? Lavender is not tolerant of too much soil moisture especially if the crown (place on the plant where the roots meet the main stem) is waterlogged. Many people make the mistake of planting the crown at or below soil level when it needs to be much higher. "We plant up on mounds, not down in holes" is something of a mantra for us. And while lavender can be divided and transplanted easily in the winter, it doesn't like roots being disturbed during the growing season. I'm also not sure about the sunlight...I'm in zone 9 as well and lavender is a full sun 6-10 hours a day type of plant. It may have succumbed to a combination of factors....but I'm betting a waterlogged crown was the mail culprit.