Too much or too little water or a nutrient deficiency. Here are quotes from experts on the subject, I hope it helps answer your question:

From R. Campbell and J. Crane about this subject -

Crane, Jonathan H:
Molecular
cause usually lack of calcium in cell walls causes weak cell walls.
Physiologically under relatively dry air (low relative humidity) or
prolonged drought the mango peel may stop growing or lose capacity to
expand. However, the pulp still grows/ expands and bursts the peel.
So....

I suggest:
Do not apply high analysis nitrogen containing fertilizer. Use:
1. 0-0-20 in Feb. and again May
2. 6-2-12 or similar analysis fertilizer after harvest
- low rate if no or little fruit
- moderate rate if big crop
3. Foliar micro-nutrients 2-3 times any time from April through September.
4. Soil applied iron 2 to 3 times
any time from April through September.
5.
Mature trees (>7 years old) probably only need to be watered during
prolonged dry conditions. So if we have no rain for more than 14 days
water.
6. If low pH soil apply powdered limestone 2-4 lbs in spring
or summer - takes a while to have an effect (like 6-9 months). If high
pH soil spray trees with calcium containing foliar mix in early spring.


Richard Campbell:
This is always a problem with NDM 4 when
there is not consistent moisture. We have the problem because we do not
use irrigation. If you keep the moisture more uniform with irrigation it
will help.

I've been watering consistently once a week and I fed it slow release MG citrus avo mango fertilizer in feb. I'm in socal we had a wet winter but it has been dry and low humidity since march. I'll try adding gypsum to see if it would help.