We’re not sure exactly why he was called “Loco Mateo.” Biographers think it may have been his quirkiness, or perhaps it was because of the heartbreak that he often sang about. Later studies theorize that the cause might be his singing repertoire, which included martinete lyrics that said “A mi me llaman el loco porque siempre voy callao” (“They call me crazy because I always go quiet”). No matter the reason, it’s quite fitting that the man called “crazy” would go on to invent bulerías, the palo whose name is derived from the word to make fun of oneself or cause a raucous. 

Mateo de las Heras Carrasco Vargas, a.k.a. “Loco Mateo” was a Gitano (Roma) singer from the Santiago neighborhood of Jerez. He is known as one of the greatest interpreters of the seguiriyas and soleares palos. When he sang por soleá (short for soleares), he would finish his singing with a remate (or close) that sped up the tempo. These closings are the roots of what we now know as bulerías. 

Source/further reading: https://sites.google.com/flamencodemarchena.org/rinconflamenco/el-loco-mateo