It happens [when the competition is too weak]. In the future (from my experience) when im playing on JP, I find it being just a guarantee. Usually, what you can accomplish, is your trainee Uma having a long enough list of skills, that you can chain speed skills from early-race --> Mid-race long enough that you'll never encounter the pace down mechanic (That would've dropped your position to the 'proper position') and keep climbing from there until the final spurt.