We present the results of molecular dynamics (MD) simulations of coarse-grain 1,2-di-n-alkanoyl-sn-glycero-3-phosphocholine Langmuir monolayer systems. Two separate alkanoyl length lipids are considered. Surface pressure versus area per lipid isotherms are obtained for both lipid species and are compared to experimental and previous atomistic MD work. The short-tail lipid cannot support a surface pressure above that expected at the critical micelle concentration. In contrast, the long-tail lipid is studied in the metastable regime of slightly negative surface tension. At still higher surface coverage, the interface is destabilized, leading to monolayer collapse via a novel mechanism in which a charge-neutralizing bridge of zwitterionic headgroups is established to transport lipids across the alkanoyl region. Area per lipid compression simulations are also performed at constant lipid number to demonstrate that the observed instabilities are generic.