If you increase the length of the fork, you raise the front of the bike. When you raise the front of the bike you decrease the head angle. When you decrease the head angle without increasing fork offset, you increase trail (the distance between the ground's intersects with the tire's point of contact and the steering axis. When you increase trail, straight line stability increases to the point where the bike feels like you're pulling a trailer in a crosswind. The rear wheel swings wide, so you steer into it, which makes it swing wide in the other direction, so you steer into it, . . .