why do some ipad homepage icons have black backgrounds

A couple of the icons on my ipad have black backgrounds

1. What causes this ?

2. Why are these few the only ones affected in this way ?

3. Does anything need to be done to correct this ?

4. If so, what exactly needs to be done to correct this ?

I am using an iPad A1474, which I have used for 2 or 3 years.

Any help would be appreciated.

Thank you.