That's not much of a link, though. Many factors could affect the altitude stability in that manner. The link is that the model of aircraft is new and two have crashed in a relatively short period of time.
An airliner would never display those characteristics in normal flight. For it to do that unplanned suggests something seriously wrong.
Both aircraft struggled to gain and maintain altitude.
Both had large swings in vertical speed and corresponding swings in horizontal speed.
Both reported errors that would trigger the MCAS software.
Both occurred late in the takeoff phase, above 5000ft but below 10,000ft.
Both led to an unrecoverable event that crashed the aircraft.
You look at the altitude and speed graphs of the two flights and its almost identical.
There is a huge safety culture in aviation and all the above is easily enough to warrant the groundings. At this point it says more about the US regulators being reluctant to upset American interests than the rest of the world being swayed by public perception.