By the way, I thought cross spiders always sit at the center of their webs
Orb-weaving spiders (like 'your' Araneus diadematus) often don't sit in the center of their webs to be less visible to predators and prey. Instead, they retreat to a hidden place at the edge of the web. A signal thread ("trap line") runs from this concealed spot to the center of the web. When an insect hits the web, the vibrations run along this thread, alerting the spider.