It sounded like a good idea at the time, right? Move the moist bathroom air into the attic and it will simply flow out through the vents. Why go to all the trouble of venting the air all the way outdoors?
What you may not have thought about is that when warm moist bathroom air encounters cooler attic air, the moisture condenses on exposed surfaces. In places where winter weather brings snow and ice, you can expect the moisture to condense and accumulate as ice. After a long cold winter, the accumulated ice will thaw all at once.
Avoid venting any type of moist air into the attic. Proper ventilation is key to keeping your home dry and healthy.