Interesting to see that adding a useless drop down card type selection has a positive effect. Would be curious to see A/B results for that aspect alone.
Also, enough things seemed to have changed at once that its hard to measure what the true impact of any one change is.
Now, if they are sustaining 15% in 6 months, and it drops if they roll back, then I'd be sold.
Guessing game: I guess that the lock symbol and random GoDaddy SSL cert image were the most important factors.