It's because of shell filling--- when you add electrons they go into discrete orbits similar to those of the H-atom, and at higher Z, all these discrete levels bind tighter to the nucleus. There is also screening, so that the outer shells see a reduced charge, but this is in combination with shell filling. The shells are at a certain distance from the nucleus. When you fill a shell, you have the highest ionization energy, because the outermost electrons are all seeing a high nuclear charge, less screening from the core, and are in a lower level. When you add one more electron to a filled shell, it is loosely bound since it sees an effective core charge of 1 unit. There is nothing qualitatively mysterious in the ionization energy graph after these screening and shell filling are understood.