Ever since Robert Bruce's victory at the Battle of Bannockburn in 1314, the Scots had taken the initiative in the wars with England, raiding deep into the north of the country repeatedly and with comparative ease to attempt to force the English to the peace-table.
It seems unlikely that Bruce had much confidence in Lancaster, who referred to himself as 'King Arthur' in his negotiations with the Scots, but he was quick to take advantage of the threat of civil war in England.