Her extravagant lifestyle and being a foreigner.
Marie Antoinette was notoriously self-indulgent. She spent tons of French money on dresses, parties, and trinket items. She did this while her husband, Louis XVI (16), raised taxes and continued to send France into extreme poverty among the lower classes.
Marie Antoinette was a queen of a political arrangement between France and Austria. She was greatly distrusted as an outsider and was not liked by the French people. She didn't help her situation by living isolated from the people and rarely leaving the palace.