This is roulette by the way, I have this kind of experience, not that I'm winning big, but I have played before wherein me and my buddies are in a streak then suddenly they've change the lady dealer and so after that, we lost after one spin with the new dealer and I told them that we better stop and take the profits and that's what we did.
The topic title would sound paranoid if it weren't funny.

Dealers change (suddenly) because they have a schedule. If I remember correctly, it's 20 minutes, after which they take a (legal) break for another 20 minutes. Dealers aren't robots and can't work (note: playing roulette is work for them, not entertainment, as it is for gamblers like you) nonstop, so they need rest.
The casino can exploit this as a loophole and replace "weak" dealers with "stronger" ones if one of the gamblers is extremely lucky. In theory, this could be exploited, because I've heard stories of roulette dealers bragging about being able to throw the ball in a certain zone and cell (with sufficient experience and skill).
But personally, I'm inclined to think that dealer changes have more to do with their work schedule than with the casino's malicious intent to interrupt your winning streak.