It's one of three things:
1) After Jan 6th, and bringing out the national guard in such force, they were flat out hoping something, anything would happen, that they could use to pounce.
2) they heard chatter, from somewhere, and wanted to pre-empt
3) there never was noise and they were hoping to create it via power of suggestion
In DC they needed a reason to claim the low crowds for Biden. Between C19 and fear of running into Trump supporters they had it.
If Biden was smart, he would have cancelled the public parts of the inauguration, due to C19 exposure, done it inside in chambers, then done his speach on TV. Problems, all of them, solved.
