It's probably because the speed is too low and it is almost stalling. Once I takeoff, raised the gear and started to get a good climb, if the VS is too high when I press CMD, VNAV and LNAV, the 757 speed drops way below the setting I have entered in the IAS/MACH Indicator. The 757 can come close to falling out of the sky when that happens.

The best thing to do after taking off, is to lower the nose so that the speed climbs to about 180-200 knots, THEN press the CMD, VNAV and LNAV.