Can confirm, but both times it was a screw or something in my tire so I presume someone dropped some at a point of my commute.

I'd expect flats on cars to be correlated. People tend to buy tires in sets, so age related factors affect them all. Similarly they tend to get driven on the same roads, so are subjected to similar environmental damage.

Also, people are good at noticing patterns that don't exist, so that's a possibility too.