Undermining (sic) the primaries of the other party have been going on for decades. Members of both parties in open primary states have been voting for candidates in the other party that they feel is easier to beat for several cycles. Why is this different? More coordinated, certainly, but not different. The only difference this time around, as far as I can tell, is that the people that received the nomination were crazier than they have been in the past. That makes it riskier from the D perspective but, again, not different.
If you're upset about it, stop running crazy ass candidates in your preferred political party.