These are both Adenovirus vectored vaccines. My theory is that plain Adenovirus also causes clots, but likewise at percentages so low you would never see them in data. Unless you infected millions of people with the same strain, like these vaccines do.

There's studies dating back many years showing that adenoviruses can cause clots and thrombocytopenia in rodents.

The lesson might end up being, surprisingly, that mRNA vaccines are both safer and more effective than traditional vaccines