may be you had subscribed to some services like cloud mining or faucet or could be the reason the someone had pasted your btc address instead of his own (May be due to mistake). well how lucky you are
Well, it is mostly like that, because there is a lot of people who are always making mistakes on sending bitcoin to someone, just a simply typo will make your bitcoin fade away and will appear on someone's bitcoin wallet, also, it might a FAUCET that you have been doing before and is auto-withdrawing the money directly to your bitcoin wallet address, especially if you have many referrals, you could earn even though you already stop doing FAUCETS.
You don't have to worry OP about the bitcoin that is anonymously sending by anyone or just from external account, unless the bitcoin in you wallet will start vanishing or decreasing everyday in no reason, this time, you should worry because it is not normal. Just be thankful that you are receiving bitcoin in your wallet, that is a grace from above.