On what pretext did the Pelishtim stop up all the wells that Avraham had dug?
Rashi: They claimed that they would supply an attacking army with water. 1
Why is the significance of the double expression, "They stopped them up and filled them with earth"?
Ohr ha'Chayim: They also envied and hated Avraham. When he died, they closed the wells (by placing stones on top of them). And now, in their burning jealousy of Yitzchak, they filled them with earth. 1 This is why Yitzchak had to re-dig them (See 26:18).
Ha'amek Davar: Some they stopped up, and the others they filled with earth.
Oznayim la'Torah: The reason they did this is so that Avraham's name should forever be removed from the wells.
What is the significance of the fact that, although HaSh-m instructed Yitzchak to live in Eretz Pelishtim, he did not query HaSh-m when he was subsequently forced to fight for the wells that he dug?
Sanhedrin 111a: HaSh-m said to Moshe, 1 'Woe - those who are lost cannot be replaced (the Avos)! Many times I revealed Myself to them with the name 'Kel Shakai,' but they never queried My Midos or asked my name. 2 I told Yitzchak, 'Sojourn in this land (Pelishtim), yet when his servants wanted to drink water they did not find - because the Pelishtim quarreled with them... ; but he did not query My Midos!' Ohr ha'Chayim - HaSh-m caused this, to test Yitzchak.
See Rashi to Shemos 6: 1-9.
Why did the Pelishtim 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
QUESTIONS ON RASHI
Rashi writes: "The Pelishtim closed off [the wells] - Because they said, 'They are a liability for us, lest armed bands come to [attack] us.'" But perhaps they sealed the wells out of jealousy towards Yitzchak, not out of fear of attack?
Gur Aryeh: If they acted out of jealousy, they ought to have confiscated the wells for themselves (as they later did in 26: 20-21); not seal them up! That is why Rashi adds that the Targum, 'Tamun,' does not mean to hide away for one's own use, but rather to block off, as in the expression, 'Timtum ha'Lev.' 1
Gur Aryeh seems to understand that this was their primary motivation, and it was not merely a pretext.