Who is Peter Hegseth and why does he rise to political prominence through his ability to support President Donald Trump?

As a Trump supporter, I am not sure what is going on at Twitter. Just yesterday I saw a feed that was all pro-Trump but today it is all anti-Trump, why have social media ppl turned against our president?
Former Vice President Harris made a surprise appearance at a Los Angeles high school graduation ceremony. What do you think she hoped to achieve by doing so?