I spent a lot of my career pricing property-casualty insurance, so let me take a stab at this.
Insurance rates are regulated by the states. Companies must supply statistics justifying the need for a rate change. Typically, they come up with projected premiums, losses and expenses during the period the rates will be effective, based on the experience of the last 2 or 3 years, adjusted for inflation and other factors to approximate future results. Let's say the rates were going to be effective as of 1/1/2020. Of course they over-estimated the amount of losses for the effective period because the exposure (miles driven) decreased significantly due to all the shutdowns, work-from-home, etc. If they filed for a rate change as of 1/1/2021, based on experience in 2020, it would look like they needed a decrease because 2020 was such a profitable year. By giving away the windfall in the form of credits, dividends whatever, they put 2020 on a level comparable to previous years.
Dental insurance may be a bit different. I'd be interested to see the effect on claims as people develop more expensive problems because their dentist was closed or they were reluctant to risk going for cleanings after the offices opened. Of course the companies saved $$ on routine cleanings and they do protect themselves with their out-of-pocket calendar year max and their stingy % paid on major work such as bridges and crowns, but if cleanings have any preventative value, there will be more cavities and other work to be done in future years.
I'd love to be a fly on the wall as dental insurance companies discuss their requested rate changes for 2021 with their state insurance departments!