For some reason, when I use a recipricating saw to cut something, I
always had trouble at the end of the cutting.

Say a PVC pipe that is already in the ground and I made a trench to
expose it. The pipe is 2" in diameter, I exposed enough of the soil
to make sure the blade when fully extended would not cut into some
stones or pebbles or roots. I start the cut, no problem, it's ripping
through real nice and smooth, but when I am 75% of my way through, the
pipe and saw started shaking and vibrating violently to make the cut
exceedingly difficult.

Now this is probably because the enough of the pipe has been cut so it
no longer is "biting" onto the blade hard enough so the blade wanders?

Is it because I am not holding it tightly enough?

Or is this related to the quality of the saw?

Ot should I always cut something from both directions - instead of
starting a cut and carry it all the way through, cut it half way then
start the cut from the opposing side and meet in the middle?

Thanks,