This paper provides a new account for why unaccusative verbs are easier to process than unergative verbs in the reduced relative garden path construction, as demonstrated in Stevenson and Merlo [1997]. Reanalysis to the passivized reduced relative clause form requires the verb to be causative. Stevenson and Merlo [1997] argued that unaccusatives are causativized in the lexicon, while unergatives are causativized in the syntax. This account argues instead that an independently attested co-occurrence restriction contributes to greater initial ambiguity in the unergative case; causative unergatives require an argument/directional attachment of prepositional phrase [Hoekstra, 1988, Levin and Rappaport-Hovav, 1995, Folli and Harley, 2006]. We implement the unergative-PP co-occurrence restriction in Minimalist Grammars [Stabler, 1997]. We model the contribution of prepositional phrase ambiguity to unergative reduced relative ambiguity with Entropy Reduction [Hale, 2003]. We obtain greater Entropy Reductions for the unergative condition, modeling that human comprehenders are more taxed by compounded ambiguity.