Why the next episode is locked
Episodes are worked in order. Episode 3 opens once episode 2 has finished rendering.
Work is gated in sequence: an episode only opens once the previous one has reached its final render.
The reason is continuity. Scenes often inherit the closing frame from the previous episode, and characters and locations are already locked there. Letting episodes be worked out of order produces joins that do not match, and you find out only after the credits are spent.