Normally noise (playing loud music, yelling down the phone) and smell (eating fried food in an enclosed tube carriage).
I had the strangest experience yesterday though - I was going to start a thread but thought I'd be universally told iabu so didn't bother. I'm 6mos pregnant and look about 8mos pregnant. Had on a big fluffy coat with the badge prominently displayed. Got on busy tube, no seats, fine I can stand for a couple of stops it'll empty out.
One lady sat in the priority seat is absorbed in her book - happens. And if I'm tired or feeling ill I sometimes ask if they mind moving but I was okay so didn't. Opposite her in the other priority seat is a middle aged woman who has clearly seen me, has a cats bum face on and proceeds to pull out her sandwich to eat it while watching me. Strange! Woman next to her spots me, offers seat, lost in book lady realises then and jumps up as well to give me seat, I sit down. Sandwich lady continues to stare with annoyed face.
Next stop another pregnant woman gets on and sandwich lady makes a big fuss about giving her a seat (she doesn't want one and says so), repeatedly offering etc.
Still can't figure out why. Did she think I wasn't pregnant enough or deserving enough? Other pregnant lady more petite than me so prominent bump, but honestly at this stage literally no one can think it's far not pregnant + I had badge. Maybe she felt I was less further along and didn't deserve a seat?
Fwiw cats bum face lady was white but both of us pregnant women are BAME so not a racist thing I think.
So to answer OP - people who behave strangely on public transport!