I think its a combination of what you guys have said.
1 - budget surplus means that they can add to the level of national savings and they also won't be borrowing, this means that there won't be pressures on the interest rates to rise (as may happen in the crowding out effect).
2 - Lowering tax rates has a less stimulatory effect on AD than does outright govt expenditure (this is because a reduction in taxes changes your disposable income, some of which is saved, some of which is spent...while outright govt spending is directly raising aggregate expenditure). That being said, by using the surplus (yes - possibly contractionary fiscal policy) to provide a tax break instead of having a budget deficit, AD is boosted, but to a lesser effect ---> less inflationary pressures ---> less pressure on interest rates to rise.
3 - ok, this is a rather obscure one, but perhaps still valid. By reducing tax rates, labour supply may increase since people have greater incentive to work more hours/enter the labourforce etc...This then increases AS (shifts the short-run aggregate supply curve to the right) and in doing so allows for there to be a greater level of output at a lower price level ----> less pressure on interest rates.