I think it's important to remember that people aren't becoming more LGBTQ+, it's that previous generations were so oppressed that coming out was impossible.
It's common to see these kinds of graphs used in some twisted ways. People are being allowed to be themselves. We have a historical president for graphs like this with left handedness. Once we stopped murdering, beating or ostracizing people for being left-handed, we saw "huge spikes" of people being left-handed.
The oppression in previous generations definitely is a big factor.
The second big thing is that the majority of LGBTQ+ people is bisexual. And there were a lot of bisexual people who would, in previous social climate, identify as heterosexual due to living in a heterosexual relationship.
When you dig into the data of several other surveys, it becomes even more apparent that the seemingly rising number of LGBT+ is due to the removal of stigma: Earlier surveys had shown that the group that saw the biggest growth among all are bisexual/pansexual people while the other groups saw much less growth.
In the Gallup survey, most of those identifying as LGBT+ said they were bisexual: [...] 57 percent of those declaring themselves something other than cisgender heterosexual. [...]
Again, bisexuality was age-related. Those most likely to declare themselves bisexual were the youngest adults, with each older age group less and less likely to claim bisexuality.
Research from the Williams Institute at the UCLA School of Law has similarly found that a key driver of the growth in the LGBT community has been a surge in bisexual women and girls. Bisexual women make up the largest group of LGBT adults — about 35 percent, according to a Williams Institute analysis of data from three population-based surveys. More than one in 10 U.S. high school youth identifies as lesbian, gay or bisexual. And among them, 75 percent are female and 77 percent identify as bisexual. [...]
Kerith Conron, research director at the Williams Institute, said more research is needed to understand this pattern. But, she said, “my theory would be it’s more acceptable for girls to identify as bisexual. The policing of young people is particularly pronounced for boys, to be masculine,” Conron said. “And for girls, to be bisexual isn’t necessarily perceived as a significant deviation from femininity.”
So people are much more open to talk about their sexual desires for people of the same sex once the stigma is removed, but - unfortunately for young men - that is less often the case for them, so they won't admit these feelings.
I think the pandemic had a big impact as well. Social distancing meant that people were isolated from the forces that usually made them perform cisheteronormativity, and a lot of people realized "oh hey I'm queer actually".
I know quite a few "COVID queers", including myself.