No, your eth holding doesnt go down with market. This has nothing to do with it. Probably, you made a transaction for eth or an erc token..and some eth was deducted as gas fees. Otherwise, there should be no reason why you Eth would go down.
yes, I agree with what you say, if no one hacks your wallet, the only other reason is the cost of gas, how come if you do not lose your eth transaction by itself, or the biggest possibility of that could have hacked the wallet you.