When I was 18, (many moons ago)
I use to tie up my dog outside of the store.
I took my dog everywhere with me.
One day a worker came up to me, and
told me, I should never leave my dog tied out because
someone might steal her.
I was very naive and extremely honest.
It never occured to me back than that anybody would do something like that.
I never left my dog unnattended again.

Possible reason's:
1:As for the roller blades/bike riders. Dogs also have
a prey drive and like moving objects.
2: Not all dogs like strangers.
Think of it. Why do people think that dogs love
everybody? Do you like every stranger you meet?
Or like you suggested something/someone from the past
could have scared your baby.

What ever the reason's.
Give yourself and your pup a big hug and
think about doing what is best for your best
furry friend.