Why would my tax owed change by $54 on a $187 ordinary dividend, which is a 29% effective rate? I am in the 24% bracket. Isn't 20% top 1099 DIV tax?

Might depend on how much of that dividend is also "Qualified" .

The portion that solely ordinary could have jumped two or 3 fields on the tax tables...which is not strictly a % basis.  Because of the way the tax table part works in blocks of income, a $1 increase can sometimes cause a $12 increase in taxes.   (87,799 to 87,800 of taxable income results in $12 tax increase for s single person...but then doesn't change until they hit 87,850)

And if you are retired and getting SS income too, the extra income may have increased the % of SS income that is getting taxed.  

So a possible combination of things could be in play