Mid term losses are standard, but lose of majority doesn't have to be. You have to win big enough to be able to lose some of the margin of your majority without losing your majority. Otherwise you really don't have a governing coalition nationwide.
It's part of the wider aspect of why politics in the US is broken. When a party is at a point where it's own supporters are so conditioned they can't even think in terms of absolute majority wins as a reasonable target, you have party politics that aren't working.
There are no doubt a lot of reasons - but whatever the causes, the symptoms are all there to see. It apparently isn't possible to construct a centrist party in the US anymore. The money isn't there, the energy is all at the fringes, primaries (and now social media) are too easily controlled by the extremes, we know all the diagnoses. So far we can't figure out the cure.