i don't follow UK politics like that. at the time i thought it was a good move because the tories were polling over labor bigly and she took over from cameron who resigned so she needed some credibility since she wasn't in the job because the people voted for her party (with her as its leader).
She was 23 points ahead of Jeremy Corbyn when she called the election. It should have been a cakewalk.
But she made a lot of blunders, bad manifesto, she skipped leadership debates, she came across as robotic and ill prepared in interviews.
Also the conservatives ran a campaign seeking to vilify Jeremy Corbyn as socialist, soft on terror and out of touch.
Meanwhile Labour (Corbyn's party) ran a campaign on issues, with a manifesto full of stuff people would want (even if they didn't define how it could be paid for)
They also organised and organised and organised, and got young people to turn out.
You found that people, in turn, rejected the politics of fear and voted for labour in greater numbers than many predicted.
Not enough to win, but more than enough to embarrass Theresa May and potentially destroy her in politics.