Why? Why?

Raj and Avni's wedding was called off by Raj - the day of the wedding he sent his family over to say he was no longer interested in getting married to Avni as he changed his mind and decided Avni was not right for him. 

Rumors/spoilers indicate that Bhavna asked Raj not to marry Avni (Bhavna had extracted a promise from Raj a few weeks ago and leveraged that) - we the viewers are yet to see this scene so we are unaware of what Bhavna said to Raj - and how much of an explanation she gave him for asking him to do so.

It is clear that Bhavna's past and her desire to keep it secret/protect Avni and her family would lead her to do so - however what exactly happened in Umargaon - what was Bhavna's role - how she's connected to Raj's father Vikram and Raj's mama - Veer/Virat has not been disclosed.

We only see bits and pieces but there are WAY too many loopholes/gaps in coherence for us as viewers to draw any "logical" conclusion.

As far as Bhavna's past goes we know the following:

- she was originally Sangeeta and then the ill fated Umargaon incident caused her to leave her village and change her name to Bhavna

- she has been disowned by her family - they consider her dead to them

- we constantly see flashbacks of young Bhavna/Sangeeta being kicked out with a file

- she does secretly keep in touch with her family or at least tries too since she got mehendi from her maternal home during Akshat/Arpita's wedding (as per a custom) 

- recently we see flashbacks of Bhavna/Sangeeta being chased by villagers and a noose used to hang people to death

- in that incident/due to that incident Vikram lost his entire family/Bhavna may have lost her father

- Veer/Virat is a slimy fellow - he is linked to the Umargaon incident and Bhavna clearly despises him

There are a lot of gaps in the Bhavna's past history storyline - at the very beginning of the show Bhavna said that her's and Suket's marriage was arranged and she selected him via a parrot - that parrot picked Suket's pic from the prospective grooms picture bunch

Bauji somehow found Sangeeta - heard her story and renamed her to Bhavna and stated she was an orphan and had her marry Suket. 

Suket has no clue of Bhavna's past as Sangeeta

Bhavna will often speak to different people connected to umargaon from a secret diary of phone numbers and introduce herself as "Bhavna" - we must assume she's informed them of her new name.

There is alot of speculation as far as what happened - what the relationship is between Bhavna/Vikram/Veer and the mystery of that night in Umargaon - and how this will affect Avni's future more like it already has.

Here are my 2 cents on the matter - I think the CVs did not have a well planned back story when they first started it - initially it looked like it would just be a foreshadow of why the K family is against love marriages - perhaps Bhavna had a failed love affair and realizes the pain caused by putting your love life/yourself before your family and how once you go down that path its hard/impossible to turn back and re-establish those ties that have been severed (indian society/mentality) and how with generations and time and changing society no matter how much you try to close yourself off from love/romantic relationships - love finds a way hence "Aur pyar ho gaya" and though sensibilities change - the essence of love does not - perhaps how its expressed or treated is different. 

Then they decided to shift that and tie Bhavna's past with Raj's family (since they delayed showing his parents for SO long) but that is now twisted and complicated to where there really is no coherence.

What upsets me is that they went to such extremes - showing the wedding preparations and leaving Avni at the altar especially if Bhavna did orchestrate that - what kind of a mother does that?! Sure let my daughter get tagged as a being dumped at the altar - just so my secret stays a secret - what could she have done that was SO BAD?

Sorry for the rant - and really long post to your rather simple question - hope that helps! Do share your thoughts!