Correct use of たら


Whilst I think I understand the purpose of this grammatical device, I'm
confused by its usage in the following sentence:

雑誌の占いコーナーを読んでいたら、「日本人は占いが好きです」と、外国人の友達が話しかけてきた。

たら I thought was for expressing conditions or situations that might happen -
but from this sentence it sounds like, the second clause definitely DID
happen. And as such the たら would appear to take on more of a 'when' function -
as in 'when this (definitely) happened.... this happened, rather than 'if this
happened/happens... this would happen. Was my previous expectation of what it
was supposed to be correct?

By the way this is the first sentence of a passage so whilst I would love to
give as much context for the quote as possible to aid people trying to answer
the question, there's not a lot more I can do here.

