Why were the SpaceX Crew-1 astronauts backed up by guards with automatic weapons?
A NASA crew launch is a highly-visible symbol of US national pride. I mean, the slogan for the whole campaign is Launch America, and the message has always been "Launch American astronauts from American soil in an American capsule on American rockets (for the first time in 9 years at all and for the first time ever on a commercial vehicle)."
And then, these highly-visible symbolic heroic figures get into straight jackets that make it impossible for them to run, hide, duck, or cover, and strap themselves to a giant bomb!!!
That is every terrorist's dream. Terrorists want to terrorize. Challenger and Columbia were extremely deep shocks to US society, even without an actual terrorist being involved.
For this particular launch, you also have the first African-American ISS crew member launching right in the middle of highly-volatile and charged race relations. So, not only do you have the "normal" pool of "hates America" and "hates the government" terrorists, but you add white supremacists into the mix as well.
I'm not sure whether there is such a thing as a misogynist terror group, but if there is, they might get triggered by a woman astronaut who has held multiple management positions at NASA and will become commander of the ISS in April.
Is this a new thing, or were similar military guards around to guard Shuttle crews as well?
NASA KSC has their own SWAT team, and their uniform patch has the year the unit was founded on it: 1979. So, the Kennedy Space Center has had its own special operations para-military police force for over 40 years.
While they are not as well known as for example LAPD's SWAT, NYPD's ESU, the FBI's HRT, or the German Federal Police's GSG9, they consistently rank among the top teams in international SWAT competitions.
We can assume that their purpose is not to mow the lawn, so enhanced security must have been in existence since at least 1979.
In particular, the article about the KSC SWAT team on NASA.gov is in the context of the 2005 Space Shuttle return-to-flight, so at least for this particular Shuttle launch, they are explicitly mentioned in an article by a NASA journalist.
Is this the new normal, i.e. a standard procedure going forward?
Given the above, I would argue it is the old normal.