You don't think the lockdowns had anything to do with it? Fewer people out and about leading to fewer run-ins between people? Government checks for everyone, and more for those who stayed home not working?
This is likely the most significant driver. The way the table is presented makes it a bit hard to get at the key numbers.
Let us take the first two rows, and calculate the percentage of violent victimization involving a firearm.
2019-8.29%
2020-7.69%
And from this we can calculate the reduction in firearm involved violent victimization rate as (7.69-8.29)/8.29 which gives a reduction of 7.24% in relative terms.
So while that is a reduction, without having more years of data it is hard to say how significant it is, I am sure there is some year to year variation.
The third row "Rate of firearm victimization" is misleading, as it is largely driven by the decline in overall victimization, which fell by 21.59% (derived from row 1).
So the proper conclusion is overall victimization fell, and thus firearm victimization fell as well since that is a subset of overall victimization.
As to the causal factors at play, the pandemic was likely the most significant one. Its a well observed phenomenon that crime rates change with climate because as you have warmer climates people spend more time outdoors, etc. The pandemic might be expected to have the opposite effect. People being forced to stay away from each other, or choosing to of their own volition, should reduce the total amount of interaction and thus the opportunity for crime.
Other pandemic factors at play may have been the stimulus payments, since those might temporarily remove some of the economic incentives for crime.
I absolutely believe that there is a correlation between the increased number of gun owners and the reduction in violent crime. It has been almost impossible to be ignorant of the fact that over the last two years the number of first time gun owners has grown substantially. While street hoods and home invaders are evil they are still savvy. Street people are almost always savvy. They know they are now at greater risk. I also think good-guy-with-gun stories are making the news (locally) mote and more. The punks get the news in addition to the street talk. So I think they are aware that the face higher risk. I think the more goood-guy stories that get coverage the less street crime we will see.
In this case I rather doubt the increase in firearms ownership had anything to do with the reduction in crime. Crime tends to be relatively concentrated in urban areas, many of which are not areas that law abiding citizens are allowed to keep and bear arms. And while the wave of new gun owners was a diverse group, I have the feeling that the distribution is skewed towards people who were unlikely to deter any crime by means of their firearm (people who bought it and put it in a drawer, people without carry licenses, people living in suburban and rural areas that had little crime to start with, etc.)
Also generally speaking, I would not expect the decline in rate to occur the same year as the acquisitions, the effect is probabally more subtle and delayed than that.
Nevertheless, all of this has given me some ideas about data I would like to pull and numbers I would like to crunch when I have some time.
) and the biggest change I noticed at my LGS and the DC firearms registration office during the pandemic was a shift from a mostly caucasian men made up mostly of sport shooters and hunters to mostly minorities from rough neighborhoods buying glock 17s and 19s for self defense. 
