sounds like maybe you didn't get the tongue piece located properly, and it cracked after having weight applied for a while?
tough to know from your description, but I know the basic arrangement you're talking about. I've had my seats on and off a dozen times, and haven't had any trouble getting it in the right place - but you do kind of slide it forward and feel it go into place.
I rode mine home from the dealer on Thursday night, and today (Saturday) I dropped it back off with 560 miles on it for the first service.
Lunch ride to Steamboat Springs with a couple of riding buddies.
It took a long day on the bike to really be able to form an opinion on stuff so early....but man, is this thing great at covering distances at a good clip.
11,307' Berthoud Pass