We don't really know yet. It seems to be connected with the sun and CO2 variation, but it is not clear what drives that variation. Today we drive the variation, and into regions that haven't been seen for a looong time, so things are indeed different.

It's interesting that the collapse of an ice age seems to happen very fast, within a few hundreds of years, so fast changes are definitely possible.

(It's good to research this since an ice age would suck majorly for us too - and we have suppressed the next one by now.)

Milankovitch cycles combined with other geological events (think Lake Agassiz flooding into the Ocean) are what domainate natural climate variability.