About your nice query, I strongly believe:
As well as the opposite beliefs of Shia Islam (against ISIS terrorists and other similar countries who support ISIS indirectly...), they are bombing Shiite mosques and other Shi'a places (as a hidden goal) to destroy them. Why?
Since, on the whole, Shia have the characteristic of Standing against
oppressors (even oppressing in other countries, as if Shia see it as
their duty (in all conscience) to defend oppressed people), and they
are considered as big trouble for the supporter countries of ISIS (and
ISIS) where they see Shia as a serious obstacle against them (in their
ways of oppressing and plundering oppressed people (indirectly)...
Otherwise why don't you see they attack (serious attacks) to other...?! (I hope nobody mention or indicate some limited attacks or bombing of ISIS to some difference countries (except Shiite), why?
Since I strongly believe that it might be one of their negative goals to behave like that as if there is no different for them if you are Shia or other sects and religions!
But, FAIR INDIVIDUALS will easily recognize that: It is a long time that ISIS is attacking daily to Shia countries and related Shia countries (as their main targets) such as Iraq, Syria, (& Yemen) and kill oppressed children, women, ..., so, honestly, you cannot really compare these vast daily (several years) savage attacks of Isis against Shia with: One or few attacks in one or some limited days, who they attack to a few diverse countries?
Hence, fair persons can easily and insightfully infer that ISIS sometimes have some limited (e.g. 1 or 2 or 3 days) attacks --by bombing-- to some non-Shia countries to deceive the public thoughts as if all of the world is their goals ... / But please be realistic...