Monolayer-protected gold nanoparticles have been shown to embed in the hydrophobic core of lipid bilayers in a manner that resembles the behavior of transmembrane proteins. This fusion capability of the nanoparticles makes them a promising candidate for drug delivery and biological sensing applications. In recent work, we have established that stochastic lipid tail protrusions are responsible for inducing fusion of these nanoparticles with lipid bilayers. Enhancing fusion probability would hence require both an understanding of factors that govern lipid protrusions and tuning the properties of ligands that coat these nanoparticles. Moreover, understanding the mechanisms that promote lipid protrusions would also provide physical insight into more fundamental biological processes such as vesicle fusion. Using atomistic molecular dynamics simulations, we show that the relative thermodynamic likelihood of protrusion events is insensitive to lipid composition in single-component bilayers. Instead, the packing arrangement of surrounding lipids presents a steric barrier that kinetically limits the incidence of protrusion events. Through membrane surface analysis, we demonstrate that spontaneous protrusion events are facilitated by the presence of geometrical defects in the bilayer.