My crude reasoning of this election result compared to GE2011.
I think the opposition did well against PAP in GE2011, partly because of what happened in the Global Financial Crisis (GFC) and the subsequent great recession. Many people were retrenched, many people lost substantial money in investments like Lehman Bros minibonds, and many people went bankrupt. PAP did very little to help these people, especially middle-income, who had suffered during the recession. As a result many middle-income people voted for opposition in GE2011 to vent their anger, the protest votes was high at around 35%.
The near zero interest rates by central banks since GFC have fuelled an abnormal economic recovery through the explosion of debt and investments, this has created jobs and asset bubbles. Many middle-income people have since found new jobs and made some money. They have completely forgotten about the last recession, and switch to vote for PAP in this GE.
Besides the feel good factor of SG50, I suspect PAP called for the early election because Singapore could be entering another recession. Singapore job creation is stagnating, retrenchment is rising, salaries are stagnating, property prices have been dropping and the Fed could soon be raising interest rates.
In the recession, many of these people who voted for PAP could lose their jobs or lose their savings in investments, or go bankrupt, and the suffering begins again. But when they get screwed by PAP again, I will be laughing at them. They will get no sympathy from me because that's what they voted for, they deserve to get screwed by PAP.