Ok. Most likely the cause is that you are trading low volume pairs and buy orders are very low. Bittrex has very low volume lately and this can cause some dust to be acumulated.
Anyway, in my opinion, it’s not something to be worried about. I mean, if you are trading for example with 5 pairs, you are always buying and selling with the same pairs, so to have some dust (usually a few cents) doesn’t hurt, and in the case you want to stop trading in the exchange and want to clean the dust, you can do the following:
Supposed the minimum volume to trade is 0.02 ETH and you only have 0.01 ETH. Ok, you can buy another 0.01 ETH and now you have 0.02 ETH, so you can convert it back to BTC or whatever you want.
I know this is not the ideal scenario but dust is being acumulated on many exchanges, specially when trading with low volume pairs.