1 For continuing to operate to JFK airport
Uber continued to provide its service to the JFK airport after Trump's immigration ban even though other cab services announced that they aren't taking pickups to the airport, which angered some Uber users and led to creating the hashtag #deleteuber.
2 For making profit out of the protests
In addition to not stopping its service to JFK, Uber announced that it turned off the surge pricing for people who are going to the airport, which made some people think the company was profiting from the protest by making the rides cheaper.
3 Uber CEO on Trump's economic advisory council
Some people were angered by Uber CEO, Travis Kalanick, being on Trump's economic advisory council and not deciding to step down after Trump's immigration ban. This led many users to boycott the app.
4 Uber CEO response to immigration ban
Uber CEO wrote a response to the immigration ban on Facebook, which people found mundane and not supportive enough. People were angered by the fact that Uber CEO didn't clearly condemn Trump's immigration ban. (See How Uber Started?)
5 There's an alternative
People who decided to boycott Uber did so because there was another alternative to the service, which is Lyft.
6 Lyft drivers are treated better
Some people found out from Lyft drivers that they are treated better by the company than Uber drivers, which made some people decide to stop using Uber. (See Why is Lyft cheaper than Uber?)
7 Lyft's donation to ACLU
Lyft's $1 million donation to the non-profit organization, American Civil Liberties Union, that defends human rights undermined Uber's position towards the immigration ban that didn't take any action until it later decided to donate $3 mllion to help its drivers who are affected by the ban.