If it was sheltered from the past rain, then I agree with Daisy that it was probably just not quite enough water. The late winter/early spring here was generally speaking pretty warm, add the changing sun angle and stronger sun, and all of a sudden not getting enough water becomes a problem even for cacti. I'd up the watering frequency, but also make sure you have well draining soil like Daisy suggests, because you still do not want the plant to be wet too long.
These guys probably like a bit of shade in the hot summer months and once they have gotten used to a certain orientation to the sun you should maintain that, otherwise the newly exposed side of the cactus not used to the sun will sunburn quite badly.
