you come back out and start rolling or flipping
Two reasons, one is actually a bug, we've just figured it out with Gingin and Thorsten [see the posts after
https://forum.flightgear.org/viewtopic.php?f=87&t=35076&start=105#p341528]
1) During the entry (OPS 304) when pressure is high enough, roll control passed from the RCS thrusters to the evelons and the RCS thrusters are switched off. After the bounce, I am changing mode back to orbital mode (OPS 301), but the RCS roll thrusters were not properly reset and remained off. That's why, after the first pass, the Shuttle starts to roll (At some point, you see a Nasal console in which I am manually fixing the issue thanks to Thorsten).
2) But even without this issue, when you're in "orbital" mode (OPS 301), the Shuttle maintains a constant inertial attitude. So, in the Earth frame, it is necessarily spinning, this is what you see each time the Shuttle is high in orbit.
Then, at apoapsis or short after, I am putting manually the Shuttle in "good" position for re-entry, 40 degrees of angle of attach, no yaw, no roll, and switching to entry mode OPS 304. That mode maintains this attitude constant with respect to earth, which means, that in the inertial frame, the Shuttle is actually constantly changing pitch. So, in the timelapse, each time you see the Shuttle not moving, it actually spins in its inertial frame, and conversely!
Hope it helps
PS: In the second pass, the Shuttle never gets deep enough for the roll control to be passed to the elevons, and, if you pay attention, there is no random roll afterwards, just the apparent rotation due to the Shuttle be at fixed inertial position