What does "Al Taseg Gevul Olam" forbid?
Rashi: Do not retract from the custom of your ancestors. One who puts a basket under the vine at the time of harvesting [to collect grapes that drop], he steals from the poor; our verse forbids this (Pe'ah 7:3).
R. Yonah: This is a Mashal. It need not forbid theft; this Sefer does not forbid warnings written in the Torah, unless it gives the reason. Do not change from the good customs and enactments that the Avos made, and the fences that our ancestors made for the Torah. Chazal said "v'Al Titosh Toras Imecha" (1:8) applies to good customs that the Avos established (Pesachim 50b). Refer to 22:29:1:2.
Malbim: The Torah already warned "Lo Sasig Gevul Re'acha Asher Gavlu Rishonim" (Devarim 19:14). There it refers to encroaching on his colleague's border. Shlomo derived that the same applies to encroaching on every border that the Rishonim made.
Why does it say "Asher Asu Avosecha"?
R. Yonah: This is a Mashal for enactments that the Avos made, and their fences.
Malbim: Yalkut brings, if you see a custom that the Avos made, do not change it. E.g. Avraham enacted Shacharis
