As the image shows:

how this bug is triggered

One way to trigger the bug shown in the screenshot is to place a Redstone Link and a Linked Extractor (or Linked Transposer) of the same channel, and the receiver's chunk is loaded later than the sender's. For example:

• Redstone Link in chunk [#X, #Z]
• Linked Extractor in chunk [#(X+1), #(Z+1)]
  As long as there's only one sender and one receiver in the same channel, each time the world is loaded from save file, the receiver will keep inactive until the sender receives a block update (like BUD).

mechanism of triggering

It seems that the only way to activate a link receiver is in RedstoneLinkNetworkHandler.java. When called as a sender, the function RedstoneLinkNetworkHandler::updateNetworkOf will activate all its receivers, but it can't activate the receiver itself.
In most cases in a Set object with 2 members, the first one to be added is the first to be iterated. Therefore, if the sender is loaded earlier, it won't have a chance to activate the receiver.