1. If mom interferes with dad's parenting time, continually badmouths dad to the children and encourages the kids not to see their father then she can lose "child custody" or what is now known in Illinois as allocation of significant decision making. Maybe she never tells him about the children's activities such as a game he might want to see. Maybe she openly denies him his weekend because she has scheduled activities for the kids.
She can end up only seeing the children on the weekends.
2. Educational Issues. Does mom get the kids to school on time? Does she allow them to miss school? Are the children failing in school or barely passing? That can be blamed on the custodial parent.
3. Significant Others. Unfortunately, sometimes women get involved with a new partner who may be bad for the kids for a myriad of reasons from constantly drinking in front of them or constantly fighting with her in the children's presence. That new partner may even be mean to the children.
4. Financial Issues. If after the divorce, mom can barely suppport herself and the children even with child support, she may not be able to live in a safe neighborhood with adequate schools. She may only be able to afford a one bedroom apartment with three kids. Ultimately, dad can argue that he is more capable of providing for the children's needs.
These are some of the major reasons for this trend. Your attorney can guide you on not letting this happen.