The same criteria that any arbitrageur would use -- "is this trade profitable for me", and there may be many factors going into that decision. I know there are others who have a BTCST account and still hold some of my bonds, for the same purpose of arbitrage and market making. The total number of bonds in circulation doesn't actually affect each individual bond-holder.
I could alternately sell the bonds that I personally hold, put the BTC into the account, and then *not* issue extra bonds to account for it, for the same effect. But this was never intended to be a "limited-issue" bond, and I would like the number of bonds to exactly reflect the value of the BTC in the account. Or would you like me to never sell my share of the bonds at all?
I'm open to discussion about this. But I believe issuing bonds when needed is a way to keep the price of the bonds in check, without having pre-announced bond sales every week.