1) Loeb makes a bold claim about some phenomenon and attaches extraterrestrial life to it. He chooses to do this in a way that will attract media attention, like a press release.
2) The press release gets picked up by media, because Loeb is a notorious Harvard professor. At least some of the media will phrase it as "scientists say...".
3) Other scientists, perhaps at universities whose press releases are mostly ignored by media, see this and say "hey, I'm a scientist too, we do not say that". They try to engage Loeb and over time escalate their criticisms from "civil and private discussions" to "public and somewhat insulting". They might feel justified in doing this because they see Loeb's mass-media communication as being at approximately the same level of technical detail; at a certain point, a technical argument is not going to attract attention or understanding of the seriousness of the claim being made, but a blunt criticism will ("this guy is nuts and does not represent his field").
4) Loeb describes 3) as a bunch of overly conservative cranks holding back scientists and directly analogizes himself to Galileo, predictably frustrating his critics further.
Like I said, I can't say anything useful about the technical details. But it seems like an important detail here is that Loeb has repeatedly chosen to try and fight this out using public opinion, and his opponents have found themselves somewhat awkwardly picking up and using similar weapons for maybe the first time in their careers, and are not enjoying it. This seems to happen a lot in modern science!