No cause information is available.
Picture By Picture (PBP) operation puts two different personal computer sources side by side. The images can be set to PBP full which stretches both images vertically to fit the screen. The images can also be set to PBP Aspect Ratio where both images are side by side and the 21x9 aspect ratio is maintained.
Vertical Fit
Side by Side
When the video resolutions are changed, the U3415W does automatic scaling. If you are running prime mode of 3440x1440, this resolution fills the entire screen. When you change to a lower resolution, the image area gets smaller than the prime mode image of 3440x1440. At the lower resolutions, the correct aspect ratio for a given resolution is maintained.
Using the Dell Display Manager (DDM), the screen position can be changed for both images when the monitor is set to PBP or PIP. The images shrink as you lower the video resolutions and are smaller than the prime mode 3440x1440. Third-party software (AMD Catalyst for example) must configure the profiles for a quick one-click switch of resolutions.
Use the following article has more information about Dell Display Manager and how to use it: What is Dell Display Manager