1. There are probably three/many/several/varieties of reasons/causes for this dramatic/significant increase/decline in… 引起……显著增长（下降）的原因有三个（许多、几个、很多）。
2. Some reasons can explain this trend. First of all,… Secondly,… Furthermore,… 一些原因可以解释这一趋势。首先，……；其次，……；另外，……
3. Why…? For one thing,… for another,… 为什么……？第一，……;第二，……。
4. The causes for…are varied. They include…Perhaps the main cause is… 造成……的原因有很多，包括……。主要的原因可能是……
5. Perhaps the main/primary/fundamental/most important reason is… 也许，最主要（根本、重要）的原因是……
6. The reason for this is not far to seek. The first reason is that…The second one is…The third is… For all this, the main cause of…is due to… 这一问题的原因不难发现。第一个原因是……。第二个是……。第三个是……。综上所述，……最主要的原因应该是……
7. It is no easy task to identify the reasons for this phenomenon which involves several complicated factors.要找出这一现象的原因并非易事，因为它涉及若干复杂的因素。
8. Certainly,…is not the sole reason for…；…is also responsible for the change/problem.当然，……并不是唯一的原因。还有……也导致了这一变化（问题）的产生。
9. You dont have to look very far to find out/identify the reason why… 你不必花费很大的力气就会找到……的原因。
10. There are numerous reasons why…,and I would explore only a few of the most important ones here.……的原因有很多，在此我只想探讨其中几个最重要的原因。