The crime for which Frondoso was accused and arrested in the middle of his wedding was for threatening a high Commander, don Fernán Gómez de Guzmán, with a crossbow.
During the times of Fernando and Isabel, the Catholic Kings of Spain, it was common for higher powers to raid and destroy smaller villages for the sake of ransack it and keep the territory as part of a higher conquest. When FuenteOvejuna was attacked, the villagers were abused severely and it was also a common thing to expect the soldiers (such as Gomez) to take the village women.
When Gomez tryed to rape Laurencia, Frondoso cooly waited until Gomez put down his crossbow, and took it to use it against Gomez. However, he really did not threaten him verbally, but by putting the weapon straight in front of his face. This was considered an insult, and a sign of high treason, even though it was Gomez who was in the wrong.
In the end, Gomez was killed by the villagers who, instead of naming who did it when pressed to answer, choose to join in and declare that Fuenteovejuna, not just a few people, did the crime. This was a sign of strength and community that defied even the Kings and attempted to return them of their dignity.