And so, as more as higher were their weight and height, they unknowingly approacched the coffin corner, risking, among other things, to loose several thousand feet, going through the high-speed stall to low speed stall and vice versa
From Wikipedia:
As an airplane approaches its coffin corner, the margin between stall speed and critical Mach number becomes smaller and smaller. Small changes could put one wingor the other above or below the limits. For instance, a turn causes the inner wing to have a lower airspeed, and the outer wing, a higher airspeed. The aircraft could exceed both limits at once. Or, turbulence, could cause the airspeed to change suddenly, to beyond the limits.