Certified GKH Gardening Expert
This can happen when they remain in ground for too long. This should only take about 100 days to mature from planting.
If they are not that mature yet, then it could very well be a phosphorous and potassium deficiency.
This article will give you more information on the care of these: https://www.gardeningknowhow.com/edible/vegetables/rutabaga/planting-rutabaga.htm