If the game downloads content too often, it can be because you are running out of free space on your device. It's suggested you have at least 20% of free space on your device out of total for the game to perform smoothly and also to avoid frequent content downloads.

• To check your free space on your iOS device go to Settings of your device > General > About > Available
• To check your free space on your Android device go to Settings > Scroll down and touch Storage & USB

If your space is almost full, we suggest you empty some.

How and why it happens.
When you have little free space on your device, OS has to clear some as you open and use different apps. That's why Cradle of Empires content files that you have downloaded are constantly removed and that's why Cradle of Empires has to download them again and again each time you open the game.