Deliberately coded that way. You didn’t activate it by accident.
AFAIK Renoise has always randomized a BPM tempo on a new song (you’d have to ask taktik what the random range is (I get the feeling it’s randomly selecting fixed values (wholly divisible by 5) between 80-155 bpm, i.e. 80,85,90,95,100,105…145,150,155)) You can save a song template to ‘fix’ the BPM value.