Due to the characteristics of visual websites builders (aka you can place items anywhere you want on the canvas) it’s best to place items on the desktop layouts first.
The reason for this is that a mobile device layout will (in nearly all cases) end up with a much taller vertical height than the desktop.
When you place an item on the canvas in a mobile layout first, the item will automatically be placed at the same vertical height on a desktop layout. This, therefore, causes the desktop layout height to expand accordingly.
If you have created a footer (a group of items set to Follow footer) set to Show on all pages, or Show on all pages in a section, the item placed on the mobile layout will cause the desktop layout height to expand, and in some cases cause the other pages to follow accordingly, due to the footer being set to share on all pages, or pages in a section.
This is just one of many scenarios that are possible due to the many ways is which you can add content that interacts with other items, on multiple device types, and multiple pages, at the same time.
This answer is in the context of not using layout blocks, but as Mr Fozzie states, layout blocks can cause a similar effect in some cases.