According to your link the share of Income of the bottom 75% also decreased. Than it is not surprising to see their tax burden decrease.

Also, the trend seems to be the other way around: Tax rates of the top households have continuously decreased.

[1] https://www.sltrib.com/opinion/commentary/2019/10/07/david-l...

[1] https://static01.nyt.com/images/2019/10/04/us/tax-trump-weal...

[2] https://www.washingtonpost.com/business/2019/10/08/first-tim...

[2] https://arc-anglerfish-washpost-prod-washpost.s3.amazonaws.c...