She didn't cry when she said who she was nominating. She had a slight wobble in her voice for a couple of words, but that wasn't crying. Have you never felt like that when you've been put on the spot in a stressful situation and had to announce bad news to someone's face? Loads of people get a lump in their throat, what does it matter?
I think the problem is that people think humans are always in two states: not crying and crying with tears. They forget that there are many physical ways of feeling sadness, anxiety, guilt, etc. Have you never been in a stressful situation and felt your lip quivering and your cheeks getting tight? You might feel a cry coming on but manage to compose yourself or the feeling comes and goes in waves. It's entirely normal. Yinrun doesn't do it "all the time". She's not breaking down when she's participating in planned nominations privately, when she's making a cup or tea, when she's doing her exercises. She does it when she's suddenly put into unexpected emotional situations: Trish leaving, nominating Jenkin to his face, nominating Tom and Jenkin to their faces, seeing people reunited with their loved ones, hearing her boyfriend behind her. The way experiencing emotions is looked down in British culture is pathetic and the way people can't comprehend anyone being different to them in regards to this is awful.