My original question was not really asking if those low magnitude earthquakes were true precursors. Instead I was wondering why they occurred in the first place. I think that they obviously had to be related to the approaching earthquake. So what caused them to occur?

However, since you mentioned precursors I will propose the following:

There are earthquake research groups that are reportedly able to tell how much strain has accumulated in fault zones around the world. And I would guess that some of them had some data regarding that Mexico area fault zone.

So, if they knew beforehand that there was quite a bit of strain stored in that fault zone and those earthquakes began to occur in that area, then by combining those data it might have been possible for them to determine that a powerful earthquake could be about to occur in that area.

However, it could be that the various groups that know about those strain buildups are simply not watching for earthquakes like those that could be indicators that a major earthquake is about to occur.