Custody schedule just changed and my kids seem confused

So we switched from the 50/50 every other week thing to 4 days on 3 days off because my ex wanted to be closer to her new job. The kids are asking why the schedule is different and honestly I'm not sure how to explain it without sounding like I'm blaming their mom. How do you guys handle this? My 7 year old keeps asking if something is wrong with me since he's at mom's more often now.