Bellatrix had never had the attention span for long lessons or terribly intellectual conversations for hours on end. Luckily Tiberius understood this, even if he didn't care for the breaks she took from lessons when she got tired or irritated.
Either that or he knew better than to try to keep pushing a frustrated demigoddess when she's trying to do something out of her element.
It was during one of these breaks that she found the irate mob. Had Tiberius not stayed behind for the break to do whatever he did when she wandered off, she probably would have gotten no closer than to recognize the gathering as a mob. After all, the taciturn Masque knew his stuff: for him it would have been as obvious as day that these people weren't just hanging around for their health and that it would be a very bad idea to let Bellatrix anywhere near them.
Of course, being the curious and troublesome critter that she is, Bellatrix wasted no time in pushing