This was likely caused by failing LIMs or friction values on the train set to high or a combination of both.
I should explain that the LIM systems on these rides are very primitive and by modern standards considered obsolete. Modern systems can adjust to account for under performing LIM modules within a chain, where as these are basically individually triggered in timed sequence relative to a base location. If they fire, great, if they don't, well theres a few backups in there just in case. Until those fail too, in which case: