English to Spanish Social Sciences Religion Invitation to Pastors´ Conference

Hello everyone,

What does slipping back mean in the following text? I haven´t been able to associate it with "then we added"...

"During the 1950's, rates of world population growth accelerated to above 1.8% per year, reaching 2.2% per year by 1963, ***then slipping back so that in the late 1990's, we added 163 million people per year to the planet***. The 2008 rate of growth has almost halved since its height, leveling off at about 134 million people per year, which is expected to remain constant.

This text is part of an invitation to Synergize 2!, a Pastors´ Conference.

Thanks a lot for your help,
Elena