On what pretext did the Plishtim stop up all the wells that Avraham had dug?
Rashi: They claimed that they would supply an attacking army with water. 1
Just as Chizkiyahu ha'Melech stopped up the water of the upper Gichon springs (as recorded in Divrei ha'Yamim 2, 32:30).
Why is the significance of the double expression "They stopped them up and filled them with earth?
Or ha'Chayim: When Avraham died, they closed the wells (by placing stones on top of them. But now, in their burning jealousy of Yitzchak, they filled them with earth. 1
Which explains, he says, why Yitzchak had to re-dig them (See Pasuk 18).
Why did the Plishtim confine their hatred to stopping up the wells? Why did they not attack Yitzchak himself?
Seforno: Because the king had warned them in no uncertain terms, not to lay a hand on Yitzchak. 1
See Pasuk 11.
רש"י: שאמרו תקלה הם לנו מפני הגייסות: אולי סתמו את הבארות מחמת קנאה ולא מחשש גייסות?
גור אריה: אם עשו מחמת קנאה- היה להם לקחת את הבארות ולא לסתום אותם.