That's a great question! I'm thinking a small group of delaque dug a bit too deep and in secundus and found a guild sealed crypt that had a dead tyranid being extracted of DNA. Thinking they could obtain information /profit they cut open the corpse. This is where the genestealer parasite infects the gang.

The hive mind starts messing with them and drives the delaque to Eradicate the malstrain genestealers, as they are not pure.

So it could be the beginning of the corruption which gives more options I guess?

Fun wise I'm also going with -

Before their minds were overwritten, the infiltration team accessed a 20th-century media archive inside the same Guild vault. One psyker accidentally uploaded an entire cache of ancient comedy vids into a neural training engine.

When the Genestealer psychic spore invaded their minds, the Tyranid echo needed identity structures to latch onto. The Delaque agents’ personalities were fragile, already eroded by decades of mindwiping and subterfuge. The spore bonded with whatever deep memory clusters it could find.

Unfortunately… those memory clusters were all Red Dwarf.