The only time this happened to me was on the way home after a long ride in the city.
I was about 5km from home and pretty dog-tired. So I was taking it easy, going maybe 17-18 kmh on a semi-busy street.
All of a sudden I felt a hand on my back.
Now this is strange when you're out by yourself and in a city where cyclists NEVER acknowledge eachother, never say "hi", even stopped side by side at a light. Nothing -- zip, nil, nada, zilch, waloo, rien du tout.
Strangely enough though, it didn't startle me too much because it was a solid, kind, gentle sort of touch. When I turned to look, there was an older gentleman, clad in cycling gear, with a cool bike (I'm incapable of telling you what kind).
He smiled and said "Je te pousse un peu?" (You need a little push?). I smiled and said "Oui je veux bien!" (Yes, I'd like that"😉. He pushed maybe 100 meters, and then took off.
It was a really nice exchange, but I think it was all in the way he did it. It might have been different if he had been akward or seemed interested in striking up a conversation. My bet is that he had a lot of miles under his belt, and lots of experience giving fellow cyclists a hand climbing.
Voilà. And the French lesson was free of charge ;-). A side note to francophiles (the rest of you can skip this part) - he used the familiar form of you (tu) when talking to me, instead of the one you use with total strangers (vous). So I have surmised that among serious cyclists (he, not I), there is a kind of cameraderie that allows you to forego the formal title. As is often the case when you are a member of the same group. Interesting.
S