You're right, a temperature rise without a corresponding increase in water vapor content does reduce RH (relative humidity). It does not, however, remove any water from the air in terms of mass (absolute humidity). So if water vapor going into the system increases RH faster than temperature rises can decrease it, RH will continue to increase. In your case, water isn't being added to the system fast enough to outpace the changes from temperature.

According to the article I linked:

> Not surprisingly, incidents tended to cluster on coastlines along confined seas, gulfs and straits, where evaporating seawater provides abundant moisture to be sucked up by hot air. In some areas further inland, moisture-laden monsoon winds or wide areas of crop irrigation appear to play the same role.

In other words, this problem is worst where a lot of water vapor is being added to the system.