What you are seeing is the ink coalescing (commonly called bleeding) between the red and black. This means that the ink dots are not setting and are instead merging and flowing into each other, in a similar fashion as drops of rain on the roof of a car as it begins to rain. Eventually, as more drops fall, the drops merge together, gain volume, and move off of the surface in a rivulet.
The same thing is happening in the area where the red ink (really Y+M ink) and the black ink (probably K+CMY inks) are merging and bleeding together before the dots are set. To control this, you need to do one (or more) of the following:
a) Increase the temperature of the media in the print zone (increases the evaporation rate of the carrier).
b) Reduce the amount of primary ink channels (CMYK).
c) Slow down the carriage speed.
d) Slow down the media feed speed.
Depending on the RIP that you are using, you should be able to perform A,C,D without re-profiling, but B would require a new media profile be built.