Some horses just don't like being girthed up.
Some have long memories: they remember having a too-tight saddle when being girthed up was uncomfortable. Some don't like being mounted from the ground because the nearside tree point digs in as weight goes into the stirrup.
I once met a horse that went ballistic when I did the girth up - and I'm always careful because you never know - but the owner had no problems. That one once had been galled by a martingale breast strap.
Some horses know they're going to be girthed up too tight (elastic-end girths are the usual culprit. Riders don't allow for the extra 'give' and overdo it).
Then there's the occasional one which doesn't want to be ridden any longer. Rare, admittedly, but they do exist.
Girthing's often a memory trigger. You just have to treat each horse as an individual :)