That one slide out of the four is electric rather than hydraulic like the rest, although I understand a few are otherwise; yours is a similar model to ours, which is electric. So a leaking solenoid, a common culprit on the other slideouts, should not be suspect here. Others here have plenty of experience with that particular slide, and I'd speculate from their prior posts that the issue is with the slide's brake. Gravity is overcoming it.
Until repaired, you probably need to insert a 2X4 brace to keep it in.
Joel