Out to lunch today, at a rural café, I talked with an old farm mechanic. He said best he can remember, the problem is usually the rope operated level that is used to trip the tie for the last bale, may be getting activated somehow. Maybe rough ground as has been mentioned. The other possibility is the tie arm isn't always going the full distance. One fix might be to drain the hydraulic fluid, change the filter, and be sure to bleed it out completely. Maybe when it was rebuilt, it didn't get bled out.