The correct answer is He raised taxes, which angered the citizens and led to uprisings.
Indeed, he introduced the principle of direct taxes based on the person (capita) and the land (lugera). In other words, a landowner would pay taxes not only on the size of his land but on the slaves/servants living and working in it. Tax collectors called Decurions collected such tax and they were liable for the accurate amount of taxes they collected.
He actually increased the size of the army and installed the Tetrarchy, a system of four co-emperors governing one of the four administrative divisions of the Roman Empire. Attila was not even born when Diocletian was the ruling emperor.