Final answer:
John Tyler was nicknamed "His Accidency" due to his unexpected rise to the presidency after Harrison's death and his controversial policy decisions that alienated him from the Whig Party.
Step-by-step explanation:
John Tyler earned the nickname "His Accidency" because he became president due to the unexpected death of President William Henry Harrison, who died just one month after taking office. Tyler's succession was controversial as he was the first vice president to assume the presidency in this manner, and some questioned the legitimacy of his claim to the office.
Additionally, Tyler's policies were at odds with his former Whig Party; after vetoing their proposal for a new national bank twice and opposing economic measures like increased tariffs and internal improvements, Tyler effectively alienated congressional Whigs. This rupture with his party and the subsequent takeover of Congress by the Democrats in 1842 meant that Tyler was effectively a president without a party.