It is not that opposition was strong. Opposition in fact had a less good hand compared to 2011 when the ground was much, much angrier. What went well for the opposition, especially WP, was that they had attracted even more talent. Their teams in 2015 were even better compared to 2011.
The image painted (rightly or wrongly) was that the opposition was going to win and bite off more than it could chew. This scared some swing voters. However, I don't think this is the main reason. The main thing was that people believed (rightly or wrongly) that PAP had made improvements, and also were influenced by the SG50 and LKY factors.
Pioneers like Tiger Hong being lauded at LHL's NDR speech was a powerful image too. When 22 year old girls can cry at NDP at the images of LKY's achievements, you know that the ground swing will favour PAP. It was simply that both sides did not guess the swing will be that huge.