[khedoros1]

Most NES games have separate program and graphics data, stored in 2 physical ROM chips on the cartridge. The CPU and Picture-Processing-Unit (PPU) have separate memory maps; one chip will be mapped into the CPU's area, and one into the PPU's. Some games (like MegaMan and Legend of Zelda) have RAM connected to the PPU, instead of a ROM chip, and the graphics are stored with the program data. This lets the developer store (or generate, like in the game "Elite") images alongside program code, and transfer to the PPU as needed.

So I'd wonder if a pattern like that is characteristic of the game switching transmitting graphic tiles to the PPU.