I am not 100% certain of the exact instances behind the Twin Sisters gates, but I suspect the usual- timber theft, dumping, fires, shooting, long-term camping, general mayhem, etc. These days it's much easier for the timber companies to gate their ownerships than deal with the shenanigans. But, gates do get vandalized all the time and aren't cheap to maintain either, but still probably cheaper than the alternative.
I work a bit in the timber mgmt. landscape and I fully understand why the timber companies lock people out!