Companies buying massive amounts of their own shares may be a key reason why certain sectors are doing better than others in the nine-year bull market for stocks.​  

On Monday, Yardeni Research said technology companies bought back $1 trillion worth of their shares since the beginning of the bull market in first-quarter 2009, followed by the consumer discretionary sector at $632 billion.  

Apple represented about 20 percent of the technology sector's total with $199.6 billion of share repurchases in the time period, according to its website.