Chimney Swift parent climbs over chicks instead of taking off and flying

Adult Chimney Swifts inside this chimney nest usually move from one position to another by using hover flight with head up. In this case, though, the parent is seen climbing down and then over the nest to perch in a different position than before, now below the nest. We don't know why this change in position was necessary, and the chicks seem just as confused as us.