Q: iphone 6 plus landscape mode is always crash

Hello EricTseCh, 

Welcome to the Apple Support Communities!

I understand that when you have the Display Zoom set as standard on your iPhone 6 Plus, you are experiencing some unexpected restarts. To troubleshoot this situation, there are a few different steps that I would suggest working through. 

First, make sure you have a backup of your iPhone in either iCloud or iTunes. If you need information on how to back up your device, please refer to the attached article below. 

How to back up your devices using iCloud or iTunes - Apple Support

After you have a current backup of your device, I would first suggest reinstalling the iPhone Operating System (iOS). To complete this process, please refer to the instructions I have selected from the next attached article. This will only reinstall the current version of iOS and should not affect any of your data. 

If your iPhone, iPad, or iPod touch doesn't respond or doesn't turn on - Apple Support

1. Connect your device to a computer and open iTunes. If you don't have a computer, you can borrow one, or go to an Apple Retail Store or Apple Authorized Service Provider for help. 
2. While your device is connected, force it to restart. Press and hold the Sleep/Wake and Home buttons at the same time. Don't release when you see the Apple logo. Keep holding until you see the Recovery mode screen.
   
3. When you get the option to restore or update, choose Update. iTunes will try to reinstall iOS without erasing your data.
   

iTunes will download the software for your device. If it takes more than 15 minutes, your device will exit recovery mode and you'll need to repeat steps 2 and 3 again.

If after reinstalling the operating system, you continue to have an issue with your iPhone restarting with the standard size display, please refer to the next attached article for information on how to proceed. 

If your iOS device restarts or displays the Apple logo unexpectedly - Apple Support

Have a great day