I believe the suit is based on at least two subjects: "standard essential patents" and "licensed on fair, reasonable and non-discriminatory terms".
"Standard essential patents" means a standard organization established a international standard, such LTE, CDMA, that requires certain patents to be used in order to be fully complied with the particular standard. In exchange the patent holders agree to license their patents to anyone in a "fair, reasonable and non-discriminatory terms". It will make no any sense if the international standard organization, for example International Telecommunication Union (ITU), will set a standard to a allow Qualcomm a free realm to make whatever amount of money Qualcomm decided.
One thing gets Qualcomm in trouble is they demand the patent fees based on the whole handset price, not just the Qualcomm chip's price.
We all know today's smartphones provide so much features and usefulness because it can take great pictures and videos, can understand what you think and ask, can identify you by your face or fingerprints, and can run many apps to improve the life of a human being. Most of those features have little or no relationship with Qualcomm's patents. To me, Qualcomm patent license fees calculated on the entire phone's price is not a "fair, reasonable and non-discriminatory" practice.