I think the reason why they got delisted was either due to low trade volumes or due to the dev miner fee where a large percentage of a mined block was sent to the developers wallet.
From what I recall the block reward was 314 MUSIC and later it was reduced to 250 MUSIC where the difference went to the dev wallet.
This created an issue because it pretty much was no different than having a premine and generally coins which have premine are similiar to ICOs and most of those were labelled as securities and since Bittrex is in USA they need to comply with SEC.
Most likely if they never did the forced dev block miner fee, then they would of remained listed.
https://coinmarketcap.com/ delisted the musicoin also
CMC usually delists coins when there is no active market for them. Many reasons for doing this. One way is to keep the market cap accurate and if a coin stopped trading at a certain price then the market wouldn't be fully accurate since you can't actually sell that coin at that price.
Also a coin could be delisted at a high price related to BTC, and BTC can rally like it did from $3k to $14K and the alt would get market cap even though the price isn't accurate, that's another reason why they delist.
And I don't think its a real delist, most likely when an exchange gets added, it will be listed again.
