One simple reason would be that major shareholders had indicated...

One simple reason would be that major shareholders had indicated to the NWE BOD that they will accept the offer.
MIN would have talked to most of the major shareholders BEFORE launching a takeover bid.
NWE BOD would have definitely talked to them after.

For a large holder it is very hard to sell their shares without depressing the market.
We can sell a few million in one or two parcels.
They cannot sell 30 or 40 million plus shares in the same way.
For a large shareholder this was a great opportunity.
They received shares in a company with a lot more liquidity.
They can sell MIN a lot more easily than NWE.