Funny you should ask, because I just happened to see Robin Hanson share [0] the exact statistics on this today.

2020: 64

2019: 11

2018: 13

2017: 9

2016: 4

2015: 4

[0] https://twitter.com/robinhanson/status/1346656330149470211

Just glancing at the linked spreadsheet, I see way more than 4 events in 2020 for which termination is pretty clearly justified and necessary, so this trend seems to be driven at least in part by a change in behavior by professors and not just others' responses to that behavior. Also worth mentioning that the majority of the listed professors don't seem to have actually been fired or even disciplined - unsuccessful petitions and the like are included.

it's also the case that some groups have become hyper-vigilant about academic "cancellations" in the last several years, which could account for why so many purported incidents have occurred since more attention is being paid to the issue.

I don't see any evidence that this spreadsheet is the result of someone systematically reviewing every case of a professor or other academic being dismissed, rebuked or whatever since 2014, so I don't really see how it could constitute a trend.

seeing a lot of people in this list "denounced in an open letter", "petitioned for firing" and the like. maybe we have different ideas about what it means to be "cancelled".

Even outside of explicit censorship I think that the US education system has been doing a very poor job lately in terms of promoting critical thinking skills. It's too common to see folks come out of the educational system with too much faith put in "both-sides-ism" it's important to empathize and understand different viewpoints but it is lazy to accept all differing views as being equally valid.