Currency Devaluation and the causes of the Tequila Crisis
The Mexican government has a complicated history of currency intervention, which can be read about in a detailed timeline here.

However, in 1994 the devaluation was caused by the end of government intervention. Up until that date the Mexican government had a set range of USD values that the peso could float within, so a sort of flexible peg to the USD. In December of 1994 the government decided to allow the peso to float freely and the peso was devalued naturally through market forces.