This has been a recurring theme the past 18-24 months. There are a few threads on this topic.
The first thing to remember is that each pair of numbered stalls shares the 135kW charge. The first to plug in receives the maximum allowed by his vehicle given his state of charge. The second to plug in get what is left over. As the battery in the first car fills, the charge will taper down, and the rate on the second car will increase. Most Supercharger stations are paired 1A-1B; 2A-2B; 3A-3B etc. but there are a few out there that are 1A-2A-3A-1B-2B-3B.
Other possibilities that have been bantered about include worn equipment. Those plugs receive a lot of use, misuse, and abuse. There are reports that a station will receive brand new connections and the charging speed shoots up. In the hot summers, the plugs cannot dissipate the heat as easily, so Tesla might be reducing the current to avoid overheating at the point of connection. Maybe there is dirt in the connection.
A lot of these are conjecture. But it is a known fact that we are generally seeing reduced charging speeds with more frequency.
