First of all, for the benefit of all readers, THE HALF COCK IS NOT A SAFETY. If you go cranking on the trigger with the lock in half cock, it will probably go off, and may also break the half cock hook on the tumbler or the sear.
Now, it should not be super easy to make it go off at half cock, but it's definitely not going to be impossible. Some lock designs will be more prone to going off half cocked than others.
That said, if it is going off real easy, just like if it were at full cock, and it never did this before, then probably something is binding somewhere, and the sear is not fully going into the half cock notch (or it's just broken). Very possibly the trigger is putting upward pressure on the sear when it shouldn't. The wood may have expanded or contracted and moved things around (this is why the dreaded "slack" in the trigger is NOT a bad thing). Possibly there's just some junk in the way somewhere. Can't tell without having it in my hands.