txid:
e9a66845e05d5abc0ad04ec80f774a7e585c6e8db975962d069a522137b80c1d
raw transaction:
01000000010b6072b386d4a773235237f64c1126ac3b240c84b917a3909ba1c43ded5f51f4000000008c493046022100bb1ad26df930a51cce110cf44f7a48c3c561fd977500b1ae5d6b6fd13d0b3f4a022100c5b42951acedff14abba2736fd574bdb465f3e6f8da12e2c5303954aca7f78f3014104a7135bfe824c97ecc01ec7d7e336185c81e2aa2c41ab175407c09484ce9694b44953fcb751206564a9c24dd094d42fdbfdd5aad3e063ce6af4cfaaea4ea14fbbffffffff0140420f00000000001976a91439aa3d569e06a1d7926dc4be1193c99bf2eb9ee088ac00000000
Perform a SHA-256 hash on the raw transaction twice ( you can do this online using https://emn178.github.io/online-tools/sha256.html, use input-type hex):
SHA-256((0100.. 0a00) = 201fa3cb0ad2a6f14d87492a582ccaa6dc6635946b700e2ab45099d2609ae187
SHA-256(201fa3cb0ad2a6f14d87492a582ccaa6dc6635946b700e2ab45099d2609ae187) = 1d0cb83721529a062d9675b98d6e5c587e4a770fc84ed00abc5a5de04568a6e9
This means the e9 (least significant byte, at he most right should go the far left: e9....
The next byte is added to this becoming: e9a6
Completing this reversing will give: e9a66845e05d5abc0ad04ec80f774a7e585c6e8db975962d069a522137b80c1d, which is the txid expected.
Check out more info about big-endian and little-endian here: https://www.techtarget.com/searchnetworking/definition/big-endian-and-little-endian. Although that doesn't answer why Bitcoin choose to use little endian in the first place for txid's.