In the Back-to-the-Future-Verse, time travelers seem to be largely insulated from the results of their own actions. Marty, for example still has detailed memories about his family and their lives after he alters the timeline at the end of BTTF1.
It stands to reason that anything short of an existential paradox (where the traveler causes him or herself to cease to exist) wouldn't create a direct paradox. The Doc would still remember why he'd interfered in the lives of Marty's kids even if his actions had removed the original cause of his meddling.
The BTTF2 Novelisation glosses over this inconsistency with a masterful hand-wave. Marty finds the whole thing confusing and so should we. Try not to think about it too hard.
Doc pulled out his binocular card to get a better look at the
courthouse, and what looked to Marty like a robot, with a USA Today
logo on its back, taking a picture of the wreckage. Marty realised
that very photo must be the one that appeared in the new version of
tomorrow’s newspaper - the version they had right in front of them.
But that was weird. How could something change when it hadn’t happened
yet? Marty decided he still didn’t understand this time travel
business at all!
Doc tucked the binocular card back in his pocket and grinned broadly.
‘Proof beyond positive that we’ve succeeded!’ he cheered. ‘Because
this hoverboard incident has now occurred, Griff now goes to jail.
Therefore, your son won’t go with him tonight, and that robbery will
never take place! Thus, due to the ripple effect, the newspaper is now
altered!’
‘The ripple effect?’ Marty asked.
Doc nodded. ‘Just as the past affects the future, the future
reverberates into the past.’
Whoa. This was heavy. But Marty remembered something like this
happening once before, when he had first messed things up in 1955.
‘Kind of like that picture of me and Dave and Linda,’ he asked, ‘where
my brother and sister started to disappear?’
‘Precisely!’ Doc patted his young cohort enthusiastically on the
shoulder. ‘Marty, we’ve succeeded! Not exactly as I planned, but no
matter. Mission accomplished!' He took a step toward the alley. ‘Let’s
get Jennifer and go home.’
The Official BTTF FAQ also offers the following (less helpful) info. It's certainly possible that the Doc always meddled in that event, it's just that he'd forgotten about it
There's a theory (we like to call it the "Self-Preservation Instinct
of the Space-Time Continuum Theory") that says that the continuum is
always trying to keep itself "on course," and when things happen to
change it, it always tries to correct itself. It is much like a river,
which tries to keep its overall course. Although earthquakes, fallen
trees, floods, or other circumstances might disrupt it at points, the
river would cut a new channel so that it would end up back at the same
place. Thus, the overall physics (or metaphysics) of the space-time
continuum would insure that any of Doc's memories of events that might
create paradoxes would become hazy — or be erased.
