Because they were supposedly illegally imported.
As many are already aware, the Land Rover Defender hasn't been sold in the US for several years. That doesn't mean it's impossible to buy one stateside, but it's definitely more difficult. So, many hopeful Defender buyers turn to another solution: the import market. However, doing so may have its own share of problems. Jalopnik is reporting that just last week some 40 Defender owners received a friendly visit from US federal authorities, specifically the US Department of Homeland Security.
The Feds were specifically after Land Rover 90s, 110s and Defenders. Several of these Defender owners claim they bought their vehicles legally from various sellers in the US, and received all the necessary paperwork proving the importation was legal, and that the VIN numbers matched. However, it's possibly the Feds are investigating a case that's even bigger. Last year, at least 20 Defenders were seized from the home of a North Carolina chiropractor. There have been no updates regarding that case, but many in the Defender community believe this guy was running an illegal import business out of his house.
Considering that most of the LR owners whose vehicles were seized last week made their purchase in the past year or so, it's entirely possible their cars were a part of this chiropractor's "side business." We'll have more updates for you once they come in, but for now the Feds aren't commenting. There's just a lot of justifiably angry (now former) Defender owners out there.