Different Reasons of Dhamma's Decline 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? 

-Metta
Ani Sutta - have desire to learn the Dhamma as taught by the Buddha and not anyone else
Anagata bhaya Sutta - teaching not practiced in earnest and procrastination
Sugata Vinaya Sutta - monks become unapproachable and have wrong understanding
Ovada Suttas - boastful about preaching ability, lack of faith, moral shame and fear, elders not setting a good example
Kimbila Suttas - lack of mutual respect towards the teacher, Dhamma, Sangha, practice, each other, heedfulness and hospitality
Saddhamma Patirupaka Sutta - counterfeit Dhamma emerging and unspiritual people lead to decline
Appamada Sutta - heedlessness,  indolence, excessive desires, unwise attention, lack of full awareness, evil friends, neglect of the wholesome lead to decline. Opposite preserves the Dhamma.
Thiti Sutta / Parihana Sutta - Satipatthana not being practiced. Practicing preserves the Dhamma.
Mahā,parinibbāna Sutta - being anxious to learn and practice Dhamma will result in the highest attainments [and preserve it]
Vinaya Cullavagga - admission of women
etc.


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:


adhigama-antaradhāna - attainments disappear, because the practice is polluted with miss conception.
paṭipatti-antaradhāna - because attainments are disappear, the practice is neglected, hence it disappear
pariyatti-antaradhāna - because practice are disappear, the learning is neglected, hence it disappear
liṅga-antaradhāna - because learning are disappear, the monasticism (Sangha) is neglected, hence it disappear
dhātu-antaradhāna - because reverence to wards the Dhātu disappear, the Dhātu is neglected, hence it disappear
So ends the Sasana and beings the dark ages