Why do the frequencies of glaciers in the past 2.5 million years change?

In the pass 2.5 million years, the frequencies of glaciers changed. The last 700,000 years were marked with oscillations of 100,000 years, but prior to 700,000 years ago, the oscillations are much weaker and show frequencies of 41,000. What happened? Did oceans play some roles in this process?