Yup. And its a bad habit almost every Marth has. Even the great ones. Marth is such a % dependent character that people pummel while they look at their opponent's % to start up the chainthrow. Trouble is, against smart people, 1 pummel at early % (up to mid teens usually) means a grab release. One of the hardest things about playing Marth is keeping track of not only your own % but your opponent's -- its not like Fox where you want to pretty much nair no matter what the situation is; and pummeling is generally a sign that you aren't properly keeping track of the %s like you should be.
Generally watch Marth videos and ask yourself "why did that CG stop prematurely?" Despite how often people say Marth should 0-death off grabs, it rarely happens, and most grabs, even among the highest level Marths, lead to marginal % gain. Its usually because of the bad habits of the Marth player, whether it be pummeling, utilting too early, misreading a percent, gambling with a dthrow fsmash, JC during a standing chaingrab (also one of the worst habits Marths have; don't jump cancel when you stand still and grab for the chainthrow. if a Fox is at 0% and you throw him up, just press Z. not jump+Z), etc. Minimize those habits and the combo game becomes more consistent.