The first one stems from the overall design of profession nodes that became a problem with masterwork ones. Normally nodes in the open world do not offer individual loot but a global one that is generated on the first interaction. If two players interact with a node, they’ll be offered the same loot. In most cases this doesn’t happen because players simply take all items, the node disappears and refreshes after a cooldown with new loot.
In case a player decides to not exhaust a node, either by taking no items or not all of them, the node stays intact and doesn’t refresh its contents. Even outside masterwork nodes it could be annoying if players would only take refinement items out of a node. The next player to interact with the same node would only find the junk left there.
In combination with masterwork nodes however this annoyance turned into a full blown exploit. The issue has been described pretty detailed in this post. Players that didn’t like the contents of a node could just wait until someone else cleared the node and would get a fresh chance at the desired resource. And since the content of nodes is generated on the first interaction, players with the Explorer’s Guild sometimes would not even get double resources if the player that generated the loot didn’t have said boon.
This issue was patched in NW.55.20160125b.4 four months ago.
During the most recent double profession weekend in May however a second issue popped up that led to PWE quickly stopping and replacing it.
It turned out the new issue was even worse. The double weekend correctly applied to masterwork nodes so that players could get two resources instead of one. This could be two of the same type but also two different ones, in which case players were able to pick only one of the two, again not clearing the node. Switching instances and repeating the process in the next instance would refresh the node in the last one, so players were able to go back and forth farming resources with a single Explorer’s Chart.
This issue was patched with Alliances, so hopefully the professions weekend will soon return.