Great question. That's something we don't know, it seems that what Er did was not important. Er's wickedness is given as a reason for his death in order to provide a backdrop to the account of Onan and Tamar. Since Tamar was childless it was very important in that culture that she had offspring. If one was widowed it was the responsibility of the brother to give them offspring and marry them (we see this passed as an actual law in Deuteronomy, this made sure widows were cared for).
To your question- we don't know for sure. All we know that in contrast to Onan and Shua Er was evil in God's sight, meaning the other brothers were God-fearing. Again, the narrative here is not about Er- thus the author decided there was not a reason to explain what they had to consitutent "wickedness".