The negative strip gave the electroscope a negative charge. While the negative strip is still present it has the effect of forcing electrons down into the leaves. If it has enough charge it will force so many into the leaves that the plate actually has a positive charge. That is, all the excess charge, plus some of the electrons originally present in the plate can be forced into the leaves. When the negative strip is removed, the electrons will re-distribute so as to make all parts of the scope negative.
Since some electrons in the leaves will move up to the plate after the strip is removed, the leaves will become less negatively charged and as a result they will converge somewhat.