It isn't that more people are becoming Republicans, just fewer people are becoming what the Democrats feel their party should be. The Democrats have once again turned far to the left, that is not where the majority of the U.S. wants to go.
People aren't leaving the Democrats, the Democrats are leaving the people.
If the Democrats fail to stop their rapid slide to the left, their base will shrink down to those who hate the right, the socialists and marxists, the rest of the FAR left, the homosexuals, and those who vote Democrat because they or their families always have. And that last segment is rapidly dying off.
The sad part of that is the Republican party and the nation will suffer for it. Without a reasonable Democratic party as a viable alternative, the Republican party will have no serious competition, and it is the competition that makes the system work.