Yes , Scotland had raised an army.

Charles I was not in a position to make them stand down as essentially the Scottish parliament and nobility were rebelling against them.

The reason was that Charles I wanted to bring the scottish church ,which was presbyterian into line with the english anglican church both in doctrine and structure(especially the introduction of bishops as the governing mechanism of the church).

To the scottish people this was outright 'popery' and an attempt to reintroduce Catholicism to scotland and the scots when faced with a choice between King and their religion chose religion.

Charles tried to impose his authority with force which triggered the 'bishops' wars , which due to lack of funds Charles lost and was forced to recall the english parliament to raise money to pay off the scots who were occupying parts of northern england which ended up leading to the 'english' civil wars.

Part of the problem was Charles lack of knowledge about Scotland. His father had become king of england as an adult and had spent the 1st 37 years of his life in scotland so he knew the political background and religious culture as well as the main 'players' so he was aware of what he could get away with in term of changes.

Charles on the other hand left scotland when he was 4 and apart from a trip to scotland in 1633 to be crowned(which caused rumblings as he was crowned using the anglican rites not the presbyterian) basically never visited, so he was deeply ignorant of scottish politics and culture so failed to recognise the limits of what was possible.