Answer:
Dred Scott's former owner, Peter Blow died. Scott was bought by an army surgeon that goes by the name of Dr. John Emerson, who later took Scott to the free state of Illinois. Emerson moved to a fort in the Wisconsin Territory, taking Scott along after two and a half years in Illinois. He then extended his stay in Wisconsin, where slavery was also prohibited. As well as staying in Illinois, that also gave him the legal standing to make a claim for freedom.
hope this helps.
All in all, when his former owner died he was bought by someone else and moved to Illinois and the Wisconsin territory. After his owner passed away Dred Scott went and "sued" for his freedom saying that since he lived in both of these areas where slavery was illegal and prohibited he should be set free, including his wife.