Why would the output voltage of a 5V SMPS (USB charger) increase with load?
Sometimes a negative load regulation term is introduced on purpose in order to compensate for IR drop in the cable between the regulator and load. This is common in automotive or industrial systems where the cable is long and bulky, thick cabling is undesirable.

If this regulator was designed for a specific phone, it is also possible that it's making up for internal drops in phone's power circuitry. For example, if an LDO is used to generate a 5V internal rail, it's dropout voltage will go up as it's load current increases. The phone designer could compensate for that and keep the LDO out of dropout by increasing the input voltage as a function of load...