I have long had ADSL connections where the upload is slower than the download and long ago I would get into cyber battles with people who had them and did a lot of research into he performance characteristics of asymmetrical links, I suspect that cellular data is asymmetric too because the base station is a big high power system and the phone is small and low power.
Even when you are downloading in the fast direction, TCP sends ACK packets in the other direction, if the traffic in the slow direction overwhelms the slow path it will cause latency and/or packet loss for those ACK packets and cause the fast direction to become unusable.
