HDMI Simulate unplugging with Microcontroller
HPD is pulled up with a resistor in the sink with +5V from the source. The +5V also  pulls up the DDC signals.
The sink can force a replug event by pulling HPD low on its side. The source will detect this, and re-initiate the DDC discovery and linkup.
However, if the source does this (that is, pull down HPD), the sink may not detect it. And if the HPD is merely disconnected at the source, the sink will definitely not see it.
Instead, the source can initiate unplug/replug by disconnecting +5V. The sink will definitely see this and assume an unplug. On reconnecting 5V, the sink will detect that event and power up, and eventually HPD will go high via the sink-side pull-up. The host then detects HPD low-to-high and sees that, triggering the linkup sequence.