First of all, a small note - people will not get a notice if you post their username in a self post, only in a comment.

That said, it is hard to know what Frost refers to, but there are several candidates.

Sweden was basically not big nor populated enough to maintain an army at grand power levels - before the introduction of the allotment system by Karl XI, the Swedish army more or less needed a war to be funded by plunder, fire-taxing (forcing cities and hamlets to pay tribute to avoid being burned) and subsidies by allied states. The end of the 30 years' war in 1648 left the Swedish state without a war and thus unable to maintain a large army. The demobilisation of the troops had also caused a scourge of veteran ex-soldiers travelling from city to city, often in bands, comitting crimes and causing debauchery and immoral behaviour (according to the church and general morale of the time).

Considering that Sweden had recently fought Russia, Poland-Lithuania, the Holy Roman Empire and Spain as well as Denmark and was thus surrounded by enemies with possible revanchist ideas, not having an army was both a domestic and a foreign problem.

Queen Kristina had spend massive amounts of money and had given away state land in grants, pawning it off for loan or giving the income of land away as pensions and pay to artists, writers, poets, painters, favourites at court as well as payment for officers and civil servants. The peasants estate at the estate parliament, afraid of increased influence for the nobility and them carrying more of the cost of the state (since the nobility and priests were extempt from tax) pushed for a reduction of noble land onwership at the estates parliament of 1650 but were denied. In 1655, the new King Karl XI Gustav agreed - aided by the need to provide for the coming war, he retracted a lot of land grants issued to the nobility during the reign of Kristina and the regency under Axel Oxenstierna to increase the income of the state. The war thus allowed the state to reclaim land and income given away and avoided a potential bankrupcy for the Swedish state, a domestic problem that could have been most severe.

The third candidate I can think of is the social upheaval and problems caused by the adancement in power and economical influence of the nobility during this era. The power of the nobility was usually curtailed by an alliance between the King and the other three estates (burghers, priests and peasants), but since the death of Gustav II Adolf, during the regency and reign of Kristina the nobility had increased their influence and their economical Power. The war became a convenient excuse for Karl X Gustav to increase royal power and curtail the growing power of the nobility, restoring the balance between the estates of the parliament.