In many suttas the Buddha stated different reasons for the decline of the Dhamma. Why and how it happens. The different causes. Can I have the references of those suttas?
More on this see the references of: The Dharma-ending Age by Piya Tan
In addition the commentarial literature has the 5 antaradhāna which is mentioned in another answer. They are:
"Formerly, bhikkhus, the Dasaarahas had a summoning-drum. As the drum began to split the Dasaarahas inserted one peg and then another peg until in time the summoning-drum's old drumhead had disappeared and only a framework of pegs remained.
"Even so, bhikkhus, will the bhikkhus become in the future. And those discourses spoken by the Tathaagata, profound in meaning, transcendental, dealing with voidness, to these they will not listen when they are recited, they will not lend an ear, they will not set the heart upon final knowledge and will not consider that those things should be learned and mastered. But those discourses made by poets, poetry, mere beautiful words and phrases, spoken by outsiders and disciples, to these they will listen... they will consider that these things should be learned and mastered.
"Therefore, bhikkhus, I say, you should train yourselves thus:
"'Those discourses spoken by the Tathaagata profound in meaning, transcendental, dealing with voidness, to these we will listen when they are recited, we will lend an ear, we will set the heart upon final knowledge and we will consider that these things should be learned and mastered.'"
— SN 20.7