Thinks rarely go even. Not even with planning. There is always some margin of error that leaves you with some leftovers. A bit too much for current load/portion/whatever, but too little to be left behind. That little bit extra could mean extra trip or unnecessary waste depending on the situation. So it's tempting to just pick them all up.
Sometimes you see it coming beforehand and can start preparing for it. Distributing the load more evenly for the remaining trips for example. No point in carrying three full loads and one with only a fraction when you can instead do four times 3/4 load (plus that quarter of a fraction). Or even do only three trips with a smaller overload.
Sometimes it's impossible to predict or see it coming. Sometimes there is no option to do the extra trip. Or then there isn't enough left for one more load. The final one needs to carry it all, or abandon the excess behind, let it go waste. It's hard to do that decision. It feels better to suffer a little than lose even a bit of what's there.