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I have seen videos and discussions about alcohol cracking acrylic, and waterblock manufacturers reminding users to not use alcohol in cooling systems with plexiglass acrylic, but why is this so?
The question proposed as duplicate only asked for the effect, but here I already know the effect. I am asking for the reason for the effect, which is not answered in the answer to the 'duplicate' question.
If a solvent causes an amorphous polymer to craze, this is because the polymer is principally soluble in it. It diffuses into the surface, but without instantly dissolving it, as the polymer chains are still stuck in the glassy network of the substrate. The polymer swells (like a fruit gum in water), but the volume increase can only happen perpendicular to the surface.
The polymer chains thereby end up oriented towards the surface, weakening the structure. When the solvent dries off again, the chains adhere to each other again, but cannot reentangle in time. The surface has become thicker now, but as it is still the same amount of polymer, the additional volume is made up by the cracks and crazes appearing in it.
According to chemical resistance of plexiglass acrylic alcohol is not as aggressive as you mention with plexiglass acrylic. But, we have to signal that acrylics is a big family of polymers (thermoplastics or thermosets) derived from acrylic acid, methacrylic acid or other related compounds. Plexiglass is only a member of this family.
Any way, in response to your question why alcohol might cause severe crazing and cracking? It's simply because alcohol, as a good solvent, has the ability to attack the surface of acrylics and dissolve the polymer chains.
The given explanation cannot be correct, because I've watched (and listened to) an acrylic glass cracking and splitting and developing leaks after pouring in a splash of bourbon. No drying out is involved. It may be simply swelling of the plastic as ethanol penetrates; this could cause cracks if the bulk of the plastic is too rigid to stretch, and can't accommodate the swelling.
A factor I haven't seen mentioned is which processes the acrylic part was subjected to during manufacturing. If the acrylic was flame polished during post processing (common in PC water cooling parts to achieve desired aesthetics), it can significantly weaken the bonds near the surface of the material and even cause microscopic crazing to occur.
When alcohol or a similar solvent is then applied, it will rapidly dissolve the weakened bonds and cause crazing, seep into the cracks, and dissolve the polymer within the cracks. The polymer will then swell, move upwards in the channel, and potentially crack the material upon drying. This is the process described in @Karl's answer.
Anecdotally, I have used near-pure isopropyl alcohol to clean the residue off many lasercut acrylic sheets without issue, and have never encountered crazing or cracking over the lifetime of those parts. However, many people report immediate cracking when cleaning milled acrylic water blocks with isopropyl alcohol. It seems likely that flame polishing is largely to blame for the part's poor alcohol resistance.
