My standard procedure is to fill the tube with enough air that takes its own shape, put that in the half-mounted tire, and then start fitting the tire at the valve, using it to push the tube and valve base back up behind the tire's bead. I am pretty careful as I get the rest of the bead over the rim, and if the tube has the right amount of air in it, it seems to work very well in keeping the the tube in the right place. I can't say that I did that the first time as I wasn't thinking about it. But the second time it definitely had my full attention and I was very careful. Interestingly, in both cases the tire bead was blown off of the rim, blowing sealant all over the place. (I've come to really hate that stuff). It really seems to me that the bead just didn't hold the pressure.
Of course I could've made a mistake, no matter how careful I thought I was being. But I don't think so. I've never had this happen before in 35 years of biking and changing my own tires. I have gotten rims pinched by not being careful when I was younger, but not in a long time, and they didn't fail like this.

On what rims did you use the Paselas? Were they super-easy to get on and off the rims?