It is not an explanation; it is the phrase's origin.
I thought you knew what it meant and needed a translation.
Here are the definitions:
# adjective: involving deductive reasoning from a general principle to a necessary effect; not supported by fact (Example: "An a priori judgment")
# adjective: based on hypothesis or theory rather than experiment
# adverb: derived by logic, without observed facts
Note: I would translate the original phrase like this: In spite of being thought of as a safe product... You would then need to add the rest of the phrase to make a complete sentence In spite of being thought of as a safe product...many were injured.
I am an interpreter and I often get a bit stuck when I hear "a priori", but now almost always opt of "initially" or "in theory" (in certain contexts) by way of an English translation.
Hope this helps you.
Quizá yo sea un inculto, pero la verdad es que 'a priori' no es una expresión que haya escuchado mucho en inglés. Diría que suena muy formal. En español, en cambio, sí que la oigo bastante, incluso en la lengua informal.