Lumped capacitance at the output needs to be kept to a minimum to make sure proper operation of the internal one-shot architecture. The one-shot architecture is designed to stay on for a specific amount of time (approx. 10ns approximately 30ns) in the TXB0104 and TXB0304. When the device is used with heavy capacitive loads, the one-shot can time-out before the signal is driven fully to the positive rail, resulting in a large RC time constant for the remainder of the output high signal.

See Leveraging Edge Rate Accelerators with Auto-Sensing Level Shifters for further clarification on how the one-shot architecture operates.