I would imagine that once the ski season begins the Airbus will position from LBA to LGW on Saturdays for the weekend charter programme. Then back to LHR on Sunday night. That's what has happened in previous years anyway!
If you look at the timetable there is one more northbound flight on Sundays than southbound - that's how they'll get the E145 back into place for Monday.
Ultimately, loads have little to do with it - it's all about yields.