This is such an interesting question because the author in "The Seafarer" is quite cryptic in regards to how he inserts these "causes" that you desire to know so much about. The first thing that causes the speaker to return to the sea is the fact that his "heart would begin to beat" again as soon as the waves began tossing him. This gives us an image of death (while on land) as opposed to life (on the sea). The second thing that causes the speaker to return begins the spiritual dimension of the poem:
The time for journeys would come and my soul / Called me eagerly out, sent me over / The horizon, seeking foreigners' homes. (36-38)
Therefore, the speaker's "soul" causes him to return in order to experience the excitement of seeing the homes of foreign lands. To continue this example, the speaker writes further of this "soul" and "heart" as he mentions roaming toward the whale's home, the vastness of the open ocean, and even the curl of waves. Thirdly, the speaker mention a simply "longing" that "wraps itself around him" in regards to the sea. I suppose this longing could be a cause as well. Finally, and perhaps most importantly, the speaker reveals in the second half of the poem that the true cause is intensely spiritual:
Thus the joys of God / Are fervent with life, where life itself / Fades quickly into the earth. (64-66)
Therefore, it is this "joy of God" that is the true cause of the speaker returning to the sea. The ocean is the only place on the earth that the speaker can experience this and, rightly so, this seafarer spends the rest of the poem talking about just that.