Really? That shouldn't be happening. It is supposed to be every 10 lines you clear you get a new mutation window where you have to pick a new block or take a cursed row.
Or do you mean when you unlock new blocks to appear in the Mutation Window? If so they're not random but there's all different requirements for unlocking them including clearing 10 lines, clearing 50 lines, getting multiple 4x Multi's, losing the game multiple times, surviving for 5 or 10 minutes. Lots of things. When you finish a run, if you click 'main menu' there's a gallery button, in there it shows all unlocked shapes and how you unlocked them :)