What Causes Expulsion Of IUD With Vaginal Bleeding?

Had an iud expell in july. Had second inserted last Wednesday. Started cramping and bleeding Saturday. Iud fell out when I went to the bathroom Sunday (today). I am still bleeding kinda heavy. What would cause me to expell the iud? And why am I still bleeding? I am 36 yrs old after my 3rd child I had an iud for 5 yrs. I had my fourth baby this fast February.