Why is my Mango Pickle foaming with Gas after 3 days, it is beginning to overflow, even though I left 2" from the cap We made a mango pickle with green mangoes. Below is the typical recipe that was used:

20 small tender green mangoes
200g salt
2tsp. turmeric powder
2” asafoetida (rock type)
1 tbsp. peppercorns
3 tbsps. Mustard seeds
¼ tsp. fenutgreekseeds
2 tbsps. Mustard dal
100g red chilli powder
1 sp. Turmeric powder
500ml sesame seed oil


Preparation : 
Wash and wipe the mangoes thoroughly. Slit them into fours. Chop 4 or 5 mangoes into cubes. 
Sprinkle the salt and turmeric powder on all the mangoes, mix them well ands place in a plastic or wooden vessel. Keep heavy weight over the mangoes and leave them for 4 days. 
Daily, turn the mangoes gently, upside down to ensure that the natural green colour of the mangoes has changed. Drain out the grime and salt if any into a pan and boil it. Cool. 
Meanwhile remove the soft seeds from each of the mangoes carefully. Remove the black skin too if any. Should there be any hard seed, do not remove it. Next, heat a tablespoon of oil in a small pan, keep the flame low and toss in the asafoetida. Remove it after a minute. Repeat with peppercorns, mustard and fenugreek seeds. Cool the spices and grind fine. Do not use nay water. Heat the remaining oil in another pan and cool thoroughly. Now mix the chilli powder with mustard dal, turmeric powder and the ground spice, add salt (the mixture should be slightly salty). Mix some cooled oil into spice mixture. Stuff each mango with a small lump of the spice paste. Place all the stuffed mangoes in a wide mouthed jar. Pour the grime and the remaining heated and cooled oil.

Now 3 days later this pickle is foaming with gas.  How can I prevent this from happening.  What am I doing wrong.  I have seen some of your posts and you seem to have a lot of experience in this area.
This "pickle" has no acid whatsoever, which is needed to pickle preserve the mangoes.   

I can only infer that it is intended to be a fermented pickle (perhaps akin to saurkraut, given the high level of salt), where bacteria break down the sugars in the mangoes and produce acid (typically lactic and acetic), and carbon dioxide.   The off gassing of the carbon dioxide can be seen as bubbling as the mangoes ferment, and would be a normal part of the process. 

The fermentation process would normally take several days to a couple of weeks (depending on temperature, mostly, and the size of the mangoes).   Is that what your recipe suggests?

If this is not meant to be a fermented pickle, then you are seeing signs of very aggressive spoilage.