1. Our plane was crap
2. We allow bombs to be placed on planes in our country.
3. We wuz shot down.
4. Nothing to do with terrorism or the fact that we are bombing in Syria.
It wasn’t a Russian plane (or even owned, built or maintained by the Russian company, which was only leasing it).
The bomb would not have been placed in Russia but in Egypt, whence the plane took off, and was probably placed not only to take out some Russian civilians, but to destroy Egypt’s tourism industry. That place is very popular with European tourists and this could really kill it.