The healthcare stock has plummeted this year, and its earnings have been coming under pressure amid rising costs.UnitedHealth Group (UNH +0.57%) has been a top dividend stock to own for years. It has consistently provided investors with a reliable dividend, and it has made generous increases to the payout over time. This year, however, it has come under pressure as its expenses have continued to rise, and it has disappointed investors with its recent earnings numbers.The stock has been in the midst of a fre ...