Why does it say that they heard that David was anointed over Yisrael?
'Rashi': Until Sha'ul and David, Pelishtim ruled over Yisrael. Yisraelim told Shimshon "you know that Pelishtim rule over us" (Shoftim 15:11) "strengthen yourselves and be valiant, Pelishtim, lest you need to serve the Ivrim like they served you" (Shmuel I, 4:9), "all Ish Yisrael saw? and they abandoned their cities and fled, and Pelishtim came and settled in them" (above, 10:7). The seven years that he ruled in Chevron, they were not concerned. They said, perhaps he is a mere leader over Chevron. Now that they heard that he is king over Yisrael, all Pelishtim gathered. They wanted to continue to rule over Yisrael!
Malbim: Refer to 14:8:2:3.
Why did they seek David?
'Rashi': Refer to 14:8:1:1.
Radak: They wanted to fight with him. Radak (Shmuel II, 5:17) - David then composed "Mizmor Lamah Ragshu Goyim" (Tehilim 2), for all Pelishtim gathered.
Malbim (Shmuel II, 5:17): Until now, they thought that he has a Bris with Achish and fights against Malchus Sha'ul. Now that [all of Yisrael] accepted him as king, they wanted to see if he will be submissive to them and pay taxes.
Why does it say that David went "Lifneihem"?
Malbim: He wanted to invade their land first, lest they spread in Eretz Yisrael. Therefore, he went to the fortress in Midbar Ma'on, which is near their land. However, they had already spread in Emek Refa'im (verse 9), near Yerushalayim.
