One way to hold header: head + -er, the thing at the head of something. Think of the word head as meaning the front end or the top end, not only the part of a body. Whatever sits at that end, and whatever gets there first, can be a header. On a page it is the strip across the top. In a file or a message it is the part the machine reads before anything else. And when a whole person goes head end first — into a ball, into a swimming pool — that movement gets the same name.
From the word’s history
Wiktionary's entry gives header as head + -er, the -er ending coming through Old English -ere. That is exactly the shape of the modern picture: the noun names a thing belonging to the head, meaning the top or front end.
Wiktionary 把 header 記成 head 加上 -er，-er 這個字尾經由古英語 -ere 進入英文。這跟上面那張畫面正好吻合：這個名詞指的就是「屬於頭那一端的東西」，而頭在這裡指最上面或最前面。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:910372989), etymology_text: From head + -er
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