I am no expert on the biology of it, and I wish I knew more how it happens, but anecdotal evidence suggests that when the wood has been boiled vigorously the white fungus often appears. This has been the common denominator when I have talked to people who have the problem (pretty common, because most people boil driftwood when they can). It may be that salt was the limiting factor in fungus growth for you, patryuji, but I certainly don't know, and I don't think that every piece of boiled aquarium driftwood is definitely going to grow white stuff, but "when you hear hoofbeats coming down the road, you think horses, not zebras."
Here is an excerpt taken from the Floridadriftwood.com site:
About Fungus
Sometimes when an enthusiast brings driftwood home, the temptation to boil it before adding it to their aquarium becomes too great to ignore. When they do this, they sterilize the wood and leave it vulnerable to fungal colonization. If this happens, do not panic! Two things to note:
1. This fungus is harmless to both fish and plants. In fact, many fish will seek it out to eat it. Yes, it looks unplesant but boiling the driftwood again and again to get rid of it is futile. This just restarts the cycle of colonization.
2. This fungus is temporary. Once the natural bacteria in your aquarium have a chance to get a foothold, they will colonize the driftwood and out compete the fungus. The fungus will then seem to disappear; almost overnight.
