Delilah realized that at last Samson has opened his heart to her and has revealed the truth. So, rather than just saying, The Philistines are upon you, Samson!
after she has worked on Samson (as in Judges 16:9, Judges 16:12, Judges 16:14), she summons the Philistine leaders before she even starts to work on him.
18 When Delilah saw that he had told her all his heart, she sent and called the lords of the Philistines, saying, “Come up again, for he has told me all his heart.” Then the lords of the Philistines came up to her and brought the money in their hands.