Someone asked me this question, and I hadn't noticed the new Tennessee tags had the letter B at the beginning of the side with letters. A quick drive proved it to be true (from what I saw). So why do they all start with "B"?
It doesn't look like they just picked at random.
Some web searches and going down the reddit rabbit hole, I've found some reasons that make sense.
Theory - They're starting in alphabetical order. What?! Then why don't all the plates start with A? That's because they don't use vowels like A. Vowels help make words. And who wants a random word in their plate like "FAT" OR "SPIT"? So they are starting with Bs (the first non-vowel).
Fun Fact - the plates with "In God We Trust" (you have to ask for them when getting your new tags), have the letters on the right side. They plates without that phrase, have the letters on the left side.