To become empty or hollow (intransitive) — a less common/older/informal reading (うつける) meaning 'to become empty'.
古い木の幹が風雨で空けてしまった。
The old tree trunk became hollowed out by wind and rain.
貝殻の中が長い年月で少しずつ空けていった。
The inside of the shell gradually became hollowed over the years.
To relax from a tense state into a vacant or absent-minded state; to become inattentive (intransitive). This usage is somewhat archaic or dialectal.
長時間の待機で、みんなだんだん空けてきた。
After waiting a long time, everyone gradually became absent-minded.
注意を失って空けると、作業ミスが増える。
When you lose focus and become vacant, mistakes in work increase.