One thing that may be contributing to the apparent spike in fares is which instant upgrade buckets are available. There are two basic categories; KUP fares booking into P (least expensive) and YUP fares booking into A (more expensive). If P sold out between the first and second times you looked then you'll be getting an A (YUP) fare. So it's not necessarily an increase in the fares themselves, but what fares are available on what dates.