A shame they picked nginx in the first place, it has all the stats and critical features behind the paid edition. HAProxy is always a better choice for load balancing.
Besides that, it looks like the move was significantly driven by GRPC and profobuf. No surprise here, GRPC really doesn't work well over HTTP. Once a company start using the google stack, they have to move to more of the google stack to make it usable.
As for the middleboxes, using gRPC-WEB[1] allowed us to switch Desktop Client App to gRPC even behind firewalls/IDSes that do not speak HTTP/2 yet.
As for the HAProxy, Dropbox used to use (circa 2013) it specifically for loadbalancing, but we eventually replaced it with our Golang proxy. That said, recent HAProxy improvements (v2.0+) make it quite an awesome dataplane and an excellent loadbalancer!