I think Da Costa had the advantage of not having to race for the championship last year, while Frijns, Bianchi and Bird were getting more careful as the season progressed. That allowed Magnussen and Da Costa to race freely, which does help a little, the field being close helps as well.
There might also be a mental factor involved; he came in last year and won, won, won, and now he’s finding himself in a much tougher fight. That might have gotten between his ears as well and raised the pressure on him; countless people (myself included) imagined he’d walk away with this year’s title. If someone like Christian Horner, Franz Tost, Helmut Marko or even the big boss himself expressed that expectation too, that might cause him some concern.