Tesla (TSLA) shares were the weakest among the Big Six megacaps in Monday's trading, declining more than 2% and falling back below their 200-day simple moving average.Other highly-valued stocks have seen weakness lately. Fellow megacap Apple, for example, has also been testing its 200-day SMA, although it has survived that test so far.But Tesla shares are feeling pressure coming from a few directions, related to and unrelated to its core business of selling cars.Stock numbers: In Monday's session, Tesla fell back below the 200-day SMA, which sits at about $584.Last week Tesla fell below that level for the first time since March 2020, at the depth of the COVID-related selling and momentum has been a concern for a month.Shares are now down 35% from their closing high in late January and 24% lower from the recent high on April 13.Just two trading sessions earlier, on April 9, the 50-day SMA