You are probably using iCloud for Photos. Optimize Storage is probably set to ON and therefore full resolution (Large File Size) photos are saved to iCloud and Low Resolution (Small File Size) is saved on your iPad. This conserves space on your iPad. When you open a photo then your iPad quickly downloads the photo from iCloud so that it can display in full resolution. Once you close the photo then it retains the low resolution photo.
What is happening is when you are attempting to edit a photo, then it is downloading the full size photo from iCloud and therefore you are asked to connect to the internet. Your iPad's photo storage space management may have some or most photos downloaded to your iPad in full resolution and for those photos, you will not need an internet connection. iPad determines this space optimization automatically.
Axel F.