As far as I understand, BU is an unilateral hard fork, meaning they still accept, on their chain, "old style" blocks (and even Segwit blocks).  In as much as that is right, it would be extremely careless for them to fork with a very small majority, because they could be orphaned if ever their majority gets lost.  If it were a bilateral hard fork, no such danger exists of course, they could even fork at 20%.

But BU cannot fork at 51% (technically, they can, but it is suicide), because the slightest bit of loss in hash rate makes their big blocks being orphaned, maybe a few weeks from now: imagine the disaster !  All BU transactions gone, as if they never took place !

Moreover, the exchanges listing BU as an alt coin, BU would lose the only thing bitcoin has going for it: its brand name.

However, if BU forks at 75%, they are pretty sure to survive for at least a while, the time people "vote in the market".  Moreover, the OTHER bitcoin, left with only 25% of hash rate, would be in serious problems: only a block every 40 minutes, and only hope to get it fixed 2 months from now, UNLESS they hardfork too and change difficulty "by hand".  Major cluster fuck for "normal bitcoin".

All normal bitcoin users would see their wallets come to a halt until they upgrade to the latest, hardforked version of "true bitcoin", with a hard fork !

Exchanges might reconsider their stance if the "true bitcoin" is a minority chain with a lot of technical difficulties and no transactions, and BU, majority chain, happily running, and call BU "bitcoin".

So BU would kill itself with a small majority, but would have serious chances to get the brand name and kill off the old chain if they had a LARGE majority.

Which miners will never give them.