Why does a current account deficit contribute to making a country a "sitting duck" for currency speculators?

I do not think this answer is definitive, but want to point out that there are two very different regimes: pegged or floating currencies.

For a pegged currency, the central bank (or similar body) is attempting to keep the currency value pegged to an external currency or gold. (I am assuming this is a traditional peg, and not a currency board.) A current account deficit implies a capital inflow, and implies a drain on central bank reserves if the private sector is not acting to support the peg. Since the central bank cannot create whatever it is pegging at will, it would eventually be forced off the peg or to devalue. (It can absorb inflows indefinitely.) One can look at the history of the Gold Standard/Bretton Woods era to see this, or search for the literature on “rational currency runs.”

For a floating currency, the currency value can go up or down. The currency market is a market, and at least some weak version of the Efficient Markets Hypothesis applies - it is not easy to make money with simple rules based on public information (other than taking on risk premia, like owning equities for the long term). One cannot look at central bank foreign currency reserves and decide that the currency will move one way or another. Presumably, a country that loses export competitiveness will eventually have its currency lose value, but we can look at countries like the United States or Australia and see that they can support current account deficits for long periods without anything too interesting happening to the currency. That said, many people extrapolate the pegged situation to floating currencies in market commentary.

Since there’s not a whole lot of interest in articles that state “we cannot find a relationship between the current account deficit and the currency value,” it would be harder to find literature on the subject.