Tchibo/O2 selectively throttling download speeds?

[inquisitor]

I can't explain what exactly happened to you but anything from network congestition to intentional throttling could have happened. Has you signal strength been affected? Then the previously serving cell may face an outage or high load has an impact on link quality (so-called cell-breathing effect). Perhaps some other heavy users "steal" bandwidth from your serving cell or O2 have an issue on their backhaul (especially cells with an microwave uplink may suffer from weather or obstacles). Also plant growth may have an impact on the involved antennas.
Recently I've read an interesting article why network equipment suppliers are offering dynamic throttling solutions, which may be another reason why you've been throttled. I haven't heard about O2 or Tchibo in particular to throttle bandwidth before the monthly allowance has been reached, so I doubt your bandwidth limitation is linked to this.
Btw as you've expressed your interest to use Lidl on another thread please note that Lidl have an additional limitation of 500MB per day on their 5GB plan. So if you ever consume 500MB on a single calendar day you will be throttled until midnight despite you have a monthly allowance of 5GB.