Checking the [matlab]-tag, I noticed that SO says that there are 1.3k followers - a few days ago it was less than 10. Is this a bug or has the tag really become so popular?
I don't think so. I see the same explosive increase on other tags. I can think of three reasons for this:
It is a relatively new feature which is not 2 weeks old yet if I am correct, people have yet to discover it and subscribe for it. Keep in mind that SO has already over a million pageviews on a daily basis.
The amounts are not real-time and only recalculated on a daily basis or two.
They added the amount of users who have added the tag to Favorite Tags to make it look better.
It can also be a combination of any of those.