Radak: There were Yisraelim who hated David. One of them asked David for money [and forced him to pay] with false witnesses. Even though we do not find this in Sefer Shmuel, we find that one of his complaints against his haters was Chamas.

Malbim: The verse teaches how they schemed against his money. They testified about matters that David would not know, e.g. that David's father borrowed from Ploni, or sold his field (so David does not inherit it from him; the buyer gets it), so he could not contradict them or bring a proof against them.