tectumedia interview questions

The tectumedia agency was very quick and contacted me on the naxt day I applied, already confirming that I could start the next week (!). Straight after 1-hour-interview on Friday, I received a job (traineeship) offer with the starting day on the following Tuesday. Next Monday though, the agency owners changed their mind and revoked the job offer after I inquired about a few minor points in the work contract (not overhours or pay-related questions) and asked to start one week later due to my personal obligations. This was quite a surprising experience for me. I'm glad that I didn't sign the contract straightaway and gave it a thought over the weekend. The whole process seemed to be suspiciously too fast for Germany. I believe the agency owners could easily fire me after 2 weeks just like that. I've never worked there but I do understand other negative comments about tectumedia on the Internet. This is simply not professional.