I suspect this is due to a reduction in BE's capacity to 2x E175 (88 pax) compared to the E195 previous years (118 pax).
Interesting as all AGP-DSA-AGP sectors I've been on between June-September this year were at 100% capacity. I can't comment on FAO/ALC/PMI.
This of course has had an impact, however the largest decrease has come from Wizzair. This is with the loss of 2x weekly SOF, the closure of LUZ and KSC bases and a return to the A320 vs A321 on the GDN route. On top of this, the weekly TCX A321 route to PMI didn't operate this year either. The decrease was mitigated slightly by additional TUI holiday flights and new destinations. Although i do think many of these flights could have seen higher load factors and were probably impacted slightly on this by the good weather here in the UK.
Although lots can change, next summers schedule could possibly be the busiest ever for DSA. Wizzair has new routes to BUD and DEB as well increased frequencies on many existing routes, namely KTW which is going daily for most part of this period at the moment. TUI with a 3rd based 737 aircraft and a weekly 787 flight to SFB. Then there's Flybe who will release their "finalised" summer 19 schedule in the next few weeks. There's definitely a few gaps im the schedule to fill with a new route or additional frequencies on an existing route(s) if they see fit.