As other people have already mentioned it is Thor who kicks Litr the dwarf into the flames at Balders funeral.

In the Prosa Edda Balders funeral ship is to big for any of the gods to launch, so they call on the femal giant Hyrrokkin who is strong enough to launch it. That makes Thor so angry that he wants to kill her, but the rest of the gods stops him. When Litr comes to close to him Thor release his anger on the dwarf.

To understand the next part it is importent to realise that much of the nordic mythologi was only verbal for a long periode of time and first written down relativly late. Because of that the mythologi and stories have changed over time.

In several other pieces the name Litr is a refference to a giant. There could both be a giant and a dwarf named Litr. Thor is said to have killed the giant Litr and in the Skáldskaparmál also Hyrrokkin. Because of that some scholars believ that in an older version of the myth thor might have killed Litr the giant and/or hyrrokkin during the funeral, but over time the myth have been retold so instead the gods calm him down and only the kick remains.

It was Þór (Thor) who kicked the dwarf Litur on Baldur's funeral pyre. Litur slid infront of Þór and, because Þór was so melancholic because of Baldur's death of the jötunn Hyrokkin's behavior, Þór kicked Litur into the fire after having laid fire to the ship.

Source: Snorra-Edda in front of me

It was actually Þór who kicked Litur onto the ship, there isn't given a real reason besides Litur running infront of Þórs feet.

I'm not sure how if this answer goes against the subs rules, but Gylfaginning only has one sentence mentioning this so a short answer is all I can give.