Catfacing is actually a pretty common problem, but so far it hasn’t been fully understood. However, it’s common enough that scientists have developed a few theories as to what causes it.
Malformations are most prevalent in heirloom tomatoes and large-fruiting varieties, so the first (and also the easiest and least expensive) way to prevent catfacing is to grow tomatoes that are less susceptible to it.
The larger and rounder the fruit, the greater the chance of catface. So, avoid those big beefsteak heirlooms, and stick with smaller tomatoes (such as cherry, grape, plum, or icicle types) or hybrid varieties that typically aren’t affected.
It’s generally believed that catfacing occurs when there’s damage to a developing tomato blossom. This happens if there’s a string of cool nighttime temperatures (below 55°F), which result in improper or incomplete pollination of the blossom.
When the flower is stressed right after it’s pollinated, the tomato ends up puckering and bulging as it tries to grow around the damaged flower. You see this more often in early summer (or in fall) when temperatures tend to drop off in the evening.