The number of a state's electoral votes could change if the

government in the state is ruled in violation of a federal policy
-results of a state election are disputed and found to be flawed
-president orders additional or fewer electoral votes to the state
-state population has a major change compared to other states

The number of a state's electoral votes could change if the state population has a major change compared to other states.

Explanation:

The United States Electoral College is the electoral body that chooses the President of the United States at the end of each US presidential election.  

The American Electoral College was established by Article 2, Section 1 of the United States Constitution. The college meets every four years with "large voters" (delegates who are elected by the "small voters" who are ordinary citizens) from each state.

Each state has the right to a certain number of delegates in the electoral college, a number that varies according to their representatives and senators in Congress. Therefore, if in the House of Representatives a State loses members due to a percentage decrease in its population, it will also lose voters in the Electoral College.

The delegates of this college elect the president of the country. There are a total of 538 votes in the electoral college. A candidate wins the presidential election if he has at least 270 collegiate votes - even if he has a lower total popular vote than another competing candidate (as happened on five occasions: John Quincy Adams in 1824, Rutherford B. Hayes in 1876, Benjamin Harrison in 1888, George W. Bush in 2000 and Donald Trump in 2016).