In Incidents in the Life of a Slave Girl, why does Linda fear her master, Dr. Flint?
Quite simply, Linda fears Dr. Flint because he is her master. As such, Dr. Flint verbally abuses her and harasses her sexually. In regards to verbal abuse, here is a good example of his typical threats:
By heavens, girl, you forget yourself too far! Are you mad? If you are, I will soon bring you to your senses.
In regards to sexual harassment, the reader learns that Linda is given a "scanty wardrobe" as she grows. This is solely for the purpose of pleasuring Dr. Flint. When Linda figures out his purpose, Harriet Jacobs reveals it in this way:
[Linda] will become prematurely knowing in evil things. Soon she will learn to tremble when she hears her master's footfall. She will be compelled to realize that she is no longer a child. If God has bestowed beauty upon her, it will prove her greatest curse.
Note that this is the perfect quotation to prove the fear that you ask about in your question. Linda finally begins to "tremble" with fear "when she hears her master's footfall." Unfortunately, it is a typical situation in the life of a female slave that the author, Harriet A. Jacobs, helps the reader understand. What is interesting is that Linda considers this a normal part of slave life.
Linda continues dealing with this abuse until she escapes. The one thing that puts Linda over the edge in regards to escape is not concern for her own welfare but concern for the welfare of her children. Dr. Flint and his wife are finally moved into the new home on the plantation, and Linda finds out that "My children were to be brought to the plantation to be 'broke in.'" This is the last straw for Linda. She escapes first to her friend Betty's house. Finally, Linda switches to her grandmother's small attic space where she can safely observe the lives of her children (and others) while still maintaining her freedom.