Re: How can this happen? Shroud Tension
You don't provide a whole lot of information to work with! Are the chainplates connected to the new bulhead?
If so, is all the tabbing on the new bulkhead intact? What kind of tangs connect the shrouds to the mast? If it is T-ball or something like it, maybe they werent properly seated when you connected them, and then they "popped" into place when you started sailing?
I think a careful inspection of your new bulhead, chainplates and standing rigging is in order before you sail again!
1979 Santana 30 Tall Rig