it absolutely could be yeast, I'd guess because you likely stored your bud a little damper and possibly a little warmer than your friend did with his. Some people actually do this on purpose, for the sake of getting interesting new flavors and allegedly for altering/enhancing the high while also smoothing it out considerably faster than tradional jar curing would, and it definitely gets a yeastiness that can take over your buds natural flavors if you allow it to ferment for too long or too hard. They call it malawi cob or just fermented weed, and it makes for a very smooth smoke and they say it even makes it edible. It's similar to tobacco fermenting methods like perrique and iirc black cavendish where they heavily compress the tobacco and allow them to age
Typically they vaccuum seal the bud to keep out aerobic mold and compress it together into a big log to maximize surface contact for the anaerobic fermenting microbes to have access to their food and do their thing (and possibly giving them a friendlier environment by high pressure) They put it all in a warm place (typical fermenting temps, 68-112 F or so) like on top of something electronic, in a food dehydrator, etc to encourage life to happen for a day or two or more depending on heat and moisture and desired outcome, before progressivley drying it out and setting it into cooler places to arrest fermentation at the stage (the smell/flavor) they like and for long term aging to deepen and mature those scents and flavors. Much like hash it only gets better over the course of months to years, with the pros recommending at least a 3 month cure to get the best out of what you've got.
I've experimented with it a bit since the last outdoor season and it can make harsh nasty weed your buddy had in a jar for years very smooth overnight with just a bit of rehydration, compression, and heat, but it's still very much a quality in quality out ordeal. Once I tried without a sealer and left my damp, parchment paper wrapped bud log on top of my cable modem a little too long with a little too much moisture and when I opened the moldy thing it smelled exactly like brie cheese
Some of those nasty grassy mold smells too so obviously I chucked it, but I think there's some very interesting potential with alternative curing methods....perhaps even adding specific yeasts, lactic acid bacterias etc like a brewer or cheesemaker would
