Learn French: L'astrologie explique de grands mystères. - Astrology explains great mysteries.

This happens because of a specific French rule: when a plural adjective (like grands) comes before a plural noun (like mystères), the plural indefinite article des changes to de.

Would it be wrong to say des grands mystères?

In strict, grammatically correct written French, yes. However, in casual spoken French, you will often hear native speakers say des grands mystères. As a B1 learner, you should practice writing de, but do not be surprised if you hear des in everyday conversation!

What if the adjective was one that goes after the noun, like fascinants (fascinating)?

If the adjective goes after the noun, you keep the normal plural article des. You would say des mystères fascinants. The change to de is only triggered when the adjective steps in front of the noun.

Why is the adjective grands placed before the noun here?

Most French adjectives go after the noun, but grand is part of the BAGS group of adjectives (Beauty, Age, Goodness, Size) that almost always go before the noun. Here, grand relates to size or magnitude.