Exodus 14:19

And the Angel of the Lord who led the way before the hosts of Israel went and came behind them; and the column of the Cloud went from before and stood behind them: because the Mizraee threw darts and stones at the Israelites, but the Cloud intercepted them

AND THE ANGEL OF G-D JOURNEYED. Rabbi Abraham ibn Ezra commented that the angel of G-d is the great prince [Michael], who went in the cloud. It is to him that Scripture refers when it said, And the Eternal went before them. When this angel, who went before the camp of Israel, journeyed and went behind them, the pillar of cloud journeyed with him. And there was the cloud and the darkness between the camp of Egypt and the camp of Israel, but it gave light by night through the pillar of fire to Israel, as it did on other nights, in order to enable them to traverse the sea, for it was at night that they traversed it.
In my opinion, that which Scripture says, And the angel of G-d journeyed, occurred at the beginning of the night. The angel of G-d, who went before the camp of Israel alludes to the Celestial Court of the Holy One, blessed be He, which is known as the attribute of justice, [which is called] “angel” in certain places of Scripture. It was he who dwelled in the pillar of fire and went before them by night to give them light. Therefore Scripture here mentions [not the Tetragrammaton, which indicates the attribute of mercy, but] ha’Elokim (G-d), [the name which denotes the attribute of justice]. It is possible that [the word malach (angel)] is not in a construct state [meaning “the angel of”] but instead is in apposition.
Now I have seen in the Mechilta of Rabbi Shimon ben Yochai: “Rabbi Yonathan the son of Yochai asked Rabbi Shimon the son of Yochai, ‘Why is it that in all places it is written, the angel of the Eternal, and here it is written, the angel of ‘Elokim’ (G-d)?’ Rabbi Shimon answered him, ‘Elokim everywhere denotes “Judge” [literally: judgment], etc.’” The Rabbis thus alluded to that which we have said.
Thus [the angel] now journeyed in the pillar of fire from before the camp of Israel and went behind them, and the pillar of cloud from before them also journeyed and stood behind them. Thus the two pillars were behind the camp of Israel. Scripture then reverts [in Verse 20] to explain that the pillar of cloud came between the camp of Egypt and the camp of Israel, that is to say, the pillar of cloud did not intervene between the pillar of fire and the camp of Israel, but rather it interposed between the camp of Egypt and the pillar of fire. And there was the cloud and the darkness between the two camps, with the pillar of …

ויסע מלאך האלוקים, the one that was guiding the pillar of cloud and the column of fire respectively in front of the marching columns of Israelites, changed positions and moved behind the columns of marching Israelites instead.

ויסע עמוד הענן מפניהם ויעמוד מאחריהם, this was the result of the angel repositioning himself. The purpose of the angel’s move now was to create a barrier between the Israelites and the Egyptians approaching from behind. The angel created a barrier of darkness, impenetrable to artificial means of lighting up the area in front of the Egyptians. (as described in Joshua 24,7). As a result, the Egyptians did not draw nearer to the Israelites during the entire night.

ההולך לפני מחנה, in the column of fire.

וילך מאחריהם, in order to (melt) dissolve the very chunks of water which had frozen in order to enable the Israelites to cross on a dry seabed, and to turn the seabed into a slimy mass which would spell doom for the Egyptians. At this juncture there was no need for the pillar of cloud to be at the head of the marching Israelites. The dry path in the water was their guide

ויעמד מאחריהם. behind the Israelites and behind the column of fire.