There is no shortage of expensive momentum stocks nowadays. Sierra Wireless ( TSX: SW )( NASDAQ: SWIR ), for example, has fallen over 16% since the beginning of May, but there is a clearer explanation as to why it has fallen. The company's first-quarter results came in lower than expectations and the second quarter will be below expectations as well. Even after the decline, Sierra is trading at a P/CF ratio of 37 times.