Read the BeagleBone System Requirements Manual.
Section 8.3 and 8.3.1
It tells you several times about the requirement to protect the boot pins, which are common with the LCD pins.
During boot, long before the device tree has any effect, the processor must read the 100K pull up and pull down resistors on these lines, in order to know which options to use to boot.
If your cape puts any kind of load on these lines, that would modify the result of the processor trying to read the 100K resistors, then the processor will be trying to boot with some other wrong option, with the usual result of not successfully booting.
--- Graham
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