1.The reasons can be listed as follows. For one thing…For another…

　　2.What has contributed to this problem? In the first place…In the second place…

　　3.Perhaps the main/primary/most important reason is…

　　4.It is no easy to identify the reason for… which involve several complicated factors.

　　5.A number of factors could account for/lead to/contribute to/result in/be conductive to…

　　6.One may trace/attribute this problem to…

　　7.This problem may result from/ largely due to/stem from a combination of factors.

　　8.The explanation for this phenomenon involves many complex reasons…

　　9.There are a variety of reasons for this phenomenon.

　　10.There are three reasons for this.