Usage patterns
〈事〉が下り坂になるfor something to start declining
The company's performance has started to decline.
〈事〉が下り坂をたどるfor something to follow a downward trend
His popularity has been on a steady decline.
天気が下り坂だthe weather is turning bad
The weather will start deteriorating from the afternoon.
下り坂にかかるto come to a downhill stretch of a road
The mountain road starts going downhill from here.