What can be the reason for gass production in this lotion?

I made this lotion a month ago. The batch size was 500g sample. Filled in 4 flip top bottles. The tablespoon that i used for mixing the batch is sometimes used for when eating food but washed properly. All bottles are ok but only one that we are using from recently produces gas. If we open it once a day you will be able to notice that it has produced gass. The emulsion is stable.
What can be the reason for gass production?
If contamination then which microorganism produces gass?

Ingredients list
tetrasodium EDTA   0.2%
Glycerin  12%
Xanthan gum   0.3%
Citric acid 0.25%
Sodium stearoyl lactylate  0.7%
GMS   2.8%
Shea butter  0.6%
Petrolatum   4%
Dimethicone  0.3%
Rose fragrance oil  0.1%

pH 5


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