My girlfriend is in the business and it is a combination of a lot of things.
There's a lack of empty containers in China, there's a lack of ships/capacity as the traffic has spiked while companies have put shipping routes on hold for a while last year, there's Brexit which delays things as lots of ships which usually go to Europe also stop in the UK, there's Chinese New Year approaching and everyone is trying to get their orders in before China effectively closes for a while (even though that might be different this year), there has also been overcapacity in the shipping industry for years and companies have worked to consolidate and reduce that which kinda seems to have worked but now comes at a bad time for everyone who wants to ship but obviously good for the shipping industry and all of this on top of a much swifter recovery than expected....
There are probably more reasons still.
At least here in Europe more stuff is transported by train from China now but they're expensive as well and only account for a tiny amount of the totals.
If memory serves well, the lockdowns in China in early 2020 meant they lost a couple 100k TEUs of export volume. These containers are bow missing elsewhere. Lockdowns in Europe and the US and so on didn't help neither.
And we have to remember that container rates have been quite low for quite a while. So any "excuse" to increase rates is welcome. What I saw since last year is, that carriers and shipping lines are offering much shorter demurrage free periods than, say, two years ago. There was a time two weeks without demurrage fees, now that is a day or so. Containers turned into a rare commodity it seems.
My guess is that there was no slack sea cargo capacity between Shanghai and LA, so once air cargo disappeared, more cargo went by sea.
Rotterdam-Shanghai, I have no idea, timeline doesn’t fit with COVID. This article suggests collusion between shipping companies, and some train engineers testing positive de-stabilizing rail cargo:
