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Squirrels, Chipmunks, mice, Voles, Moles...all possible reasons for plant injury.
Mums can tend to be short lived and it may not be anything you have done other than the age of the plant.
These articles have more information for you.
https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/chrysanthemum-lifespan.htm

https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/wintering-mums.htm

These articles will help you.
https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/chrysanthemum-bloom-season.htm
https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/repotting-chrysanthemums.htm

Adding an inch of water applies only when there has been less than an inch of rain. If there has been rain in the preceding week, subtract that amount from 1.0 inches. An inch equals 16 gallons per square foot which is a lot. If using a sprinkler or soaker hose, sink a tuna or pet food can with straight sides until it is level with the garden soil. Time how long it takes to reach one inch; this may be several hours by soaker hose. As temps cool and days shorten, the plant needs less water. The soil should be dry down 1.5-2 inches before the next watering. Weekly is an average. Temps over 85, sandy soil, large plants and windy weather increase the need for water. If your soil is heavy clay, it will hold water longer than loam or sandy soil. Certainly, the soil should not be mucky or wet. Damp soil is what you are after. By watering heavily once a week or so, you encourage the roots to go deeper into the soil; this is important for long-term survival. If your plant is in a container, water when the top 1 inch of soil is dry. The one inch recommendation is for plants in the ground in average soil.

The mums that are often sold in autumn and are covered in blooms are highly cultivated and forced into that abnormal bloom. It seems to leave the plant exhausted. In addition, these mums are often not hardy. They are treated as annuals and tossed onto the compost pile when their glory fades. Try dead-heading and fertilizing to coax more blooms. If you have planted it, put 3-4 inches of wood mulch over the crown once the ground freezes. If still in a container, try over-wintering in a garage. Don't let it dry out as this kills the plant for sure. A gentle watering on Thanksgiving, New Year's, Valentine's Day and Easter should be sufficient. The link that follows goes to an article on container mums. https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/growing-mums-in-containers.htm

They need 4 hours of sun daily. However, you can prolong bloom by giving it bright indirect light instead of full sun.

They do need plenty of water, but try watering thoroughly then waiting till the top of the soil is dry to water again. Daily may be too much. Make sure the pot is draining well.

Go ahead and cut off the brown buds. Also, inspect the plant for pests.

https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/growing-mums-in-containers.htm

While many people treat them as annuals, with a little care you should be able to overwinter them successfully and have mums again for next fall. Here is how:

https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/wintering-mums.htm