I've seen this happen quite a few times now and it's getting incredibly annoying.
Your team makes trump. Your partner then leads trump when he knows that both opponents are void in trump! The end result is almost inevitably the opponents taking the last trick and turning what should have been a two-point hand into a one-pointer.
Sure, you do want to lead trump once your team gets control so that you can clear the opponents out of their trump. But in most cases, one trump is all that's needed. Don't lead multiple times unless you've got a hand full of trump, in which case you should have gone alone! But after that first lead, don't lead trump again (unless your partner actually showed void, in which case the trump are all in the enemy hands).
It astonishes me how many people don't know this simple rule!