Shares of Pioneer Natural Resources declined 5.6% to $155.49 on Monday after the US oil producer agreed to acquire privately held rival DoublePoint Energy for about $6.4 billion.

The DoublePoint Energy deal is Pioneer's second big purchase and the fourth multi-billion shale deal this year. In January, Pioneer closed a $4.5 billion, all-stock purchase of Parsley Energy, giving it one of the largest positions in the Permian Basin, the top US shale field.

RBC Capital Markets said it was surprised Pioneer made such a large acquisition after Parsley Energy earlier this year and that the rationale seems to be part opportunistic and part defensive.