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.