I wonder this myself sometimes, OP - but - I also know suspect that I am that Mum sometimes.
I do drop-offs on my way to work, and am in a rush, have stuff on my mind, am in my own little world once I've said goodby to DS and am heading to the car. I am also a bit short-sighted without my glasses.
I am certain I have blanked people before.
I'm also not great with faces, and so unless - even if - we've met and chatted before, I won't necessarily properly recognise people in amidst the throng of other parents.
I really hope I haven't been rude to people, but I probably have, and it's 100% unintentional. I apologise!
And I try to remember this when I get blanked by people I know!
I do exclude the weirdo couple of Mums who always blank me when I try to make eye-contact and smile at them, in spite of knowing full-well who I am, but that's their (weird) issue, not mine! 