I have to admit that I used to be terribly naive about all of this: despite being brought up in Glasgow, my parents brought me up to be blind to people's religion.
I was nearly 30 before I found out that you could "tell" if someone was/might be Jewish by their name and/or how they looked. 
I was over 30 before I found out why, in Glasgow, they so often ask which school you went to, when applying for jobs - and might ask which primary school you went to, in order to find out if you are Catholic
My dh (who I met when I was 31) is a (lapsed) Catholic who opened my eyes to some of the bigotry that still exists. :(
Having said that, I think that the parcel bombs are the work of a lone nutter - even though I think that there has been an increase in sectarianism recently.
I do think that some of the escalation has been fuelled by Lennon and the Celtic chairman going on about referees' and the SFA's supposed bias against Celtic.
That is all part of why I am so against the separate State Catohlic schools - and why ds was never allowed to support either Celtic or Rangers (he ended up supporting Glasgow Queens Park) and why we have encouraged him to play (and support) rugby rather than football.
Interestingly, the last Glasgow Warriors (rugby) game we went to, they were playing Ulster. Not only were there flags with the Red Hand of Ulster being waved, they were singing Ulster songs......
....... and even worseshock], the fans were not segregated and they were allowed to buy alcohol and take it with them to their seats. 

And there was NO TROUBLE (....despite a dubious referring decision which meant that Glasgow lost 'cos of a penalty after the clock had gone red - but that's a whole different story
)
The moral of the story seems to be that the "troubles" seem to be more linked to the type of people who follow football - or is it just that the troublemakes will find something to latch on to :( , ie that they are not true fans of football.