In general, nothing has really stuck out about one particular incident of demotivation, but the entirety of my fourth year was pretty dire. Part of it may have been a bit of mild burn out, and I should have taken some time to do something other than study, but for some reason, that year I was completely uninterested in studying. This was not a particular event, but spread over the entire year. The courses were fine, and other people were doing well.
I think that the biggest issue was that I could not see any practical application of my project. Perhaps it was also that I never really bonded with the rest of the class either, certainly not to the extent that they bonded with each other.
Thinking back on it, if I had had more freedom to follow up on things that I had been interested in (essentially computational geoscience, rather than straight geology), I think it would have gone a long way to keeping me interested. Unfortunately, none of the staff were particularly interested in going that route, so I was left to muddle through. Coupled with not realising just how badly I was actually doing in the various courses until far too late (communication from lecturers being poor), and I had a pretty dismal year, academically.
Aside from that, it was mostly on me, and is not their fault. Perhaps they could have intervened sooner, or suggested that I do something else?