something similar just happened to me this week with my economy class LF on 2 of my routes. (not on the other 10 routes)
I have a monopoly on these routes, so I adjusted the ticket prices so that the average LF is around 80% at each flight (about 8 flights per route per day). The prices have been more or less stable for a few weeks and every flight has its individual price for maximum profit.
But last week the average LF dropped by 30-40% to about 55-65% LF on every single flight (both ways) on these 2 routes. I checked the demand: It has not changed and I am even under supplying these routes. Also I still am the only airline serving these routes and there is no airline in the 3 weeks forecast on the route analysis page.
This behaviour appears very odd to me... have I done something wrong?