The construction lo que pasa es que ‘what happens is that’ is a Spanish discourse marker that was originally a pseudo-cleft construction. Before becoming grammaticalized, the verb pasar contained its full lexical meaning ‘to happen,’ but later evolved into a fixed expression losing its lexical meaning and acquiring an implicit contrastive and causal meaning. The present study aims to describe the construction’s evolution on the path of grammaticalization in relation to Traugott’s (1989) three semantic-pragmatic tendencies. In addition, a Usage-based Theory approach is employed in order to describe some of the formal aspects of the construction. Using two corpora, CORDE and Corpus del Español, all instances of the construction were located and analyzed with regard to function and usage in context. Results indicate that the construction was first used in the 16th Century and that its evolution as lexical > concessive > epistemic is in line with Traugott’s tendencies. Mechanisms of change such as chunking and phonetic reduction and loss of compositionality and analyzability, as well as increase in overall frequency are also discussed in relation to this construction, lending further support to Usage-based theory.