Why was this commercial plane highly delayed mid-flight?
If you look at the details, you’ll see the aircraft left the gate at 9:37, but only took off at 10:47.

They probably received notice of the issue in MUC pointed out by Machavity while taxiing to the runway (and then asked to park somewhere waiting for the airport to be ready for them). No sense returning to the gate for a delay like this which is expected to be relatively short.

A possible alternative (though probably not here) is they they had an issue right after gate departure which triggered a maintenance request. It happened to me once on some regional airliner which had an issue just when they spooled up the engines (or just after a few meters, don’t remember). Immediately powered down and had some part of the engine changed (or maybe just a filter or something similar) while we were waiting, took about an hour or so.