I'm tempted to offer an interpretation. "The insurrectionary upsurge" refers to the rising popularity of I@ ideas and practice in the US around the time of 2008 onward (to reduce this simply to publications, which is obviously problematic, think Fire to the Prisons, Politics is Not a Banana, various local I@ periodicals). The turn toward Marxism and identity politics is a little less clear but means the rising popularity of "insurrectionary communist" ideas and other anti-state communism that has been attractive to I@s influenced by the perceived "need to return to class analysis" (think Modesto Anarcho promoting I@ alongside of solidarity networks, IEF's translating & publishing Invisible Committee & Theorie Communiste). As for identity politics, I think it's a reference to projects like Petroleuse Press which styles itself as insurrectionary or markets itself to that niche even while their texts are statist communist.
I'd say the reason for the phenomenon is threefold. One is the reduction of insurrectionary anarchist ideas to pro-violence or militancy. The other is that identity politics--including class-based identity politics--have a very strong hold, especially in the US, and a tendency to sneak in through the window when they are kicked out the door. Third is the weakness of individuals who can be swayed by whichever trend is popular, or to put it another way, whichever has the most capital behind it.
[edit for clarity; then for italics]
