Southwest Airlines’ stock was falling Thursday, after Citigroup downgraded the stock to sell from neutral, arguing that the valuation has risen while earnings quality and free-cash-flow conversion have deteriorated.

Southwest Airlines (LUV) is lower in premarket trading, after Citi downgraded the stock to "sell" from "neutral," and cut its price target to $29.50 from $31.50. The analyst in coverage cited valuation concerns and weakening earnings quality,