What has happened to our plant? After years of cutting and making “babies”, the main plant just went crazy! It’s like another plant is busting out! Or, two plants trying to occupy the same space!
Your Dieffenbachia (Dumb Cane) is a mature plant that may need repotting so that it continues to thrive – many experts recommend that houseplants be repotted every couple of years, using a good quality soilless potting mix. Mature plants often develop multiple stems. The lower leaves appear to be yellowing, soft and limp, sometimes an indication of inadequate nutrition, sometimes inadequate watering. You could remove these leaves as part of your regular upkeep. Your local nursery should be able to advise you on an appropriate liquid fertilizer formulated for indoor foliage plants. Dieffenbachias prefer bright but diffused sunlight, which yours seems to have.
Here is a fact sheet with general advice on indoor plants that should be helpful, especially regarding repotting: https://www.missouribotanicalgarden.org/Portals/0/Gardening/Gardening%20Help/Factsheets/Indoor%20Plants21.pdf. The Toronto Master Gardeners also publishes a guide to growing houseplants: https://www.torontomastergardeners.ca/gardeningguides/beginners-guide-to-growing-houseplants-a-toronto-master-gardeners-guide/