UML sequence diagrams (SDs) are a mainstay of requirements specifications for communication protocols. Mauw and Reniers' algebraic (MRA) semantics formally specifies a behaviour for these SDs that guarantees deadlock free processes. Practitioners commonly use communication semantics that differ from MRA, which may result in deadlocks. For example FIFO, token ring, etc. We define a process algebra that is an extension of the MRA semantics for regular sequence diagrams. Our algebra can describe several commonly used communication semantics. Regular SDs are constructed from concurrent message flows via iteration, branching, and sequential composition. Their behaviour is defined in terms of a set of partial orders on the events in the SD. Such partial orders are known as causal orders. We define partial order theoretic properties of a causal order that are particular kinds of race condition. We prove any of the common communication semantics we list either guarantees deadlock free SDs or can result in a deadlock if and only if a causal order of an SD contains one of these types of race condition. This describes a complete classification of deadlocks as specific types of race condition.