。
It happened while I wasn't looking.
Literal
I [subj-が] eyes [obj-を] was-letting-go [during-間に] happened [explanatory-んだ].
目を離す ('let one's eyes leave' = look away, take one's eyes off) is an idiom for not paying attention. ~ている間に ('during the time that X was happening') marks the window. ~んだ is the casual explanatory ending.