If you don’t select any boosts when starting a match, the game will select 3 random team ups from your current inventory.
Up until recently, these randomly selected team up powers would occupy a slot for just that one match. That meant that when starting your next match, 3 team ups would be selected at random again. But something changed, and now the team ups are keeping the slots occupied as if you had selected them by hand.
The randomness gave it something of a wild card element, sometimes giving you just that extra edge you needed in certain situations, but without you having to take the effort of tediously selecting a set ‘appropriate’ for each match. (Sure, sometimes you had bad luck, but they’re an extra anyway right?)
With the same TU’s locked in semi-permanently, I seem to be using team ups a lot less.